This was CNBC's live blog tracking developments on the war in Ukraine.

Russia and Ukraine attacked each other's neighboring border regions on Sunday, with the northeast Ukrainian region of Kharkiv and the Russian region of Belgorod just over the border both coming under attack.

A day of mourning has been declared in Kharkiv on Monday after a popular leisure spot and several villages were shelled by Russian forces on Sunday, leaving at least 11 civilians dead and many wounded. Kharkiv has become the prime target for Russian forces since they launched a new offensive in the border region on May 10.

A lakeside recreation spot in the outskirts of Kharkiv city was among the areas attacked Sunday, leaving at least five people dead and 16 injured, among them two ambulance workers hurt in a second attack on the same location.

Meanwhile, air raid alarms have continued in Russia's Belgorod region on Monday morning, with the regional governor warning residents to seek shelter.

Earlier, Regional Governor Vyacheslav Gladkov said on Telegram that the town of Shebekino had been attacked by a Ukrainian drone. No civilians were injured but two apartment buildings were damaged.

The latest attack, which has not been confirmed by Ukraine, comes a day after various towns in Belgorod were targeted on Sunday, injuring 13 people. CNBC was unable to independently verify the reports.

Ukraine 'holding back' Russian troops near Bilohorivka

Ukraine said Monday that its military is "holding back" Russian troops near Bilohorivka, a rural settlement in the east of the country, which Moscow claimed earlier in the day to have seized.

"The defence forces for quite a long time today have been holding back the onslaught of the enemy, which is trying to somehow move forward in the area of Bilohorivka," Kyiv's general staff said in a Google-translated Facebook post.

— Karen Gilchrist

China's foreign minister calls for close cooperation with Russia

China and Russia should stabilize the foundation of their cooperation, said China's Foreign Minister Wang Yi after meeting on Monday with his counterpart, Russian Foreign Minister Sergei Lavrov.

Wang called on both countries to maintain the security and stability of neighboring areas, according to Reuters. The minister spoke at the meeting with his counterpart in Kazakhstan's capital, Astana.

The ministers also discussed the situation in the Middle East, Ukraine and the Korean peninsula, Reuters reported.

- April Roach

Russia's Putin speaks to Iran's interim president, Kremlin says

Russian President Vladimir Putin on Monday spoke to Iran's interim President, Mohammed Mokhber, who was previously the country's vice president, the Kremlin said in a statement.

"Both sides will emphasize their mutual desire to further consistently strengthen comprehensive Russian-Iranian interaction for the benefit of the peoples of the two countries," the Kremlin said, according to a Google-translation.

Earlier on Monday, Russian state news agency Ria Novosti had reported that Russian Deputy Foreign Minister Sergei Ryabkov said he expected all agreements previously made between the two countries to be fulfilled even after the death of Iran's president and foreign minister. The two died in a helicopter crash on Sunday.

— Sophie Kiderlin

Russia takes control of another Ukrainian settlement, destroys drones over Belgorod, its defense ministry says

The Russian Defense Ministry on Monday said it took control of another Ukrainian settlement, Bilohorivka in the Luhansk region, according to a Google-translated post on Telegram.

That was a result of "hostile activities" and allows Russia to take on more advantageous positions, the ministry said.

Separately, the ministry said attacks from Ukraine on the border region Belgorod were thwarted. Two drones and a rocket were destroyed by Russian air defense systems, the ministry said in another Google-translated Telegram post.

Belgorod is a key region for Ukrainian attacks and at least 13 people were injured in the area after strikes on Sunday.

CNBC could not independently verify battleground reports.

— Sophie Kiderlin

Russian sentenced to 25 years for military office arson plot

A court in Siberia sentenced a local man to 25 years in prison on Monday for a slew of crimes including treason and attempted arson of a military recruitment office, a Russian lawyers' association said.

Prosecutors at a military court in Novosibirsk accused Ilya Baburin of trying to burn down the enlistment office with a Molotov cocktail at the behest of an unidentified person from Ukraine.

The court also found him guilty of setting fire to a local music school, which it categorised as a terrorist act.

Baburin, identified in Russian independent media as an IT specialist, is 24 years old.

The Pervy Otdel (First Department) legal association cited his lawyer as saying there was no evidence of Baburin's involvement in the incidents, in which no casualties were reported.

— Reuters

Ukraine still controls over half of embattled Vovchansk, regional official says

Ukraine still holds around 60% of the embattled town Vovchansk in the Kharkiv region, Deputy Governor Roman Semenukha told Ukrainian national television on Monday, Reuters reported.

"The enemy continues to try, especially inside Vovchansk, to push the Ukrainian Armed Forces out of the town," he said, noting that assaults from Russia are not stopping.

CNBC could not independently verify developments on the ground.

Vovchansk has become a fighting hot spot and heavily contested town in recent weeks since Russia launched a fresh offensive on the Kharkiv region in the northeast of Ukraine. Russia and Ukraine have at times given conflicting information about the the situation on the ground, including on territorial gains and losses.

— Sophie Kiderlin

Iran's Raisi made 'invaluable' contributions to Russia-Iran relations, Putin says

Russian President Vladimir Putin on Monday said that Iranian President Ebrahim Raisi made "invaluable" contributions to the relationship between Russia and Iran, as he expressed condolences over Raisi's death.

"As a true friend of Russia, he made an invaluable personal contribution to the development of good neighborly relations between our countries and made great efforts to bring them to the level of strategic partnership," Putin said in a Google-translated message to Iranian Supreme Leader Ayatollah Khamenei, which was published on the Kremlin's website.

"I will forever retain the fondest memory of this wonderful man," Putin said, adding that the helicopter crash that led to the death of Raisi and Iran's Foreign Minister Hossein Amirabdollahian was an "enormous tragedy."

His comments came after Russian Foreign Minister Sergei Lavrov{=null} earlier on Monday said that the deceased Iranian officials would always be considered "true friends."

— Sophie Kiderlin

Russian oil refinery damaged after weekend attack from Ukraine, state media reports

Russia's Slavyansk oil refinery was damaged in a drone attack from Ukraine over the weekend, Russian state news agency Tass reported on Monday.

"There is damage. Their scale and size will be determined," Eduard Trudnev, a security official for the refinery told Tass, according to a Google translation. He added that the vulnerabilities of the refinery were established and in the process of being addressed.

The refinery suspended operations and a damage assessment was being carried out following the attack, Tass previously reported.

Roman Sinyagovsky, head of the Slavyansky district, on Sunday said that six drones fell onto the grounds of the oil refinery, according to a Google-translated Telegram post. No casualties were reported initially, he said.

CNBC could not independently verify developments on the ground.

— Sophie Kiderlin

Russia sends condolences to Iran following the deaths of 'true friends'

Russia has sent its condolences to ally Iran after it was confirmed that Iranian President Ebrahim Raisi and Foreign Minister Hossein Amirabdollahian died in a helicopter crash Sunday.

Russian Foreign Minister Sergei Lavrov said the ministers had "always been known in Russia as true friends."

"The Russian Federation sincerely expresses its condolences to the families and friends of those killed in the plane crash and to the entire friendly Iranian people," Lavrov said, state news agency Tass reported.

Russia and Iran's alliance has deepened since the invasion of Ukraine in February 2022, with Tehran accused of supplying Moscow with thousands of one-way attack drones, missiles and artillery shells for use against Ukraine. Iran denies supplying weapons to Russia.

— Holly Ellyatt

Air raid alarms sound in Belgorod again after attacks injure 13

There have been several air raid alarms in Russia's Belgorod region Monday morning, with the regional governor warning residents to seek shelter.

Earlier, Regional Governor Vyacheslav Gladkov said on Telegram that the town of Shebekino had been attacked by a Ukrainian drone. No civilians were injured but two apartment buildings were damaged.

The latest attack, which has not been confirmed by Ukraine, comes a day after various towns in the border region of Belgorod were targeted on Sunday, leaving 13 people injured.

Belgorod has become a focal point for Ukrainian attacks on Russia's border regions; Russia says one of the main reasons it has launched a new offensive in Kharkiv, northeast Ukraine (and adjacent to Belgorod) is to create a "buffer zone" to protect its border regions.

Gladkov said Ukrainian forces attacked Shebekino with multiple-launch rockets on Sunday, injuring 11 people, including three children, and damaging apartment buildings and cars in the town. To the east of the town, the village of Rzhevka was also shelled, he said, injuring two people and damaging one house.

CNBC was unable to verify the reports and Ukraine has not commented on the attacks; it rarely does when it comes to strikes on Russian territory. Both Russia and Ukraine say they don't target civilians or civilian infrastructure.

— Holly Ellyatt

Day of mourning declared in Kharkiv after 11 killed in spate of Russian attacks

A day of mourning has been declared in the northeast Kharkiv region on Monday after a popular leisure spot and several villages were shelled by Russian forces on Sunday, leaving 11 civilians dead and others wounded.

"May 20 is the Day of Mourning in the Kharkiv region," Oleh Syniehubov, head of the Kharkiv regional state administration, said on Telegram late on Sunday after a number of separate attacks.

"As a result of the shelling of the villages of Kivsharivka, Novoosynove [in the Kupiansk district] and the Malodanyliv community [on the outskirts of Kharkiv city], the occupiers took the lives of 11 people. Among the dead is a pregnant woman," he said.

Kharkiv has become the prime target of Russian attacks since its forces launched a new offensive in the border region on May 10.

A lakeside recreation spot in Cherkasska Lozova on the outskirts of Kharkiv city was also among the areas attacked on Sunday, leaving at least five people dead and 16 injured, Syniehubov said, among them two ambulance workers.

He accused Russia of carrying out a "double-tap" strike in which an initial attack is followed up by a second in order to kill members of the emergency services attending the scene. Russia says it does not target civilians. In a separate attack, one person was killed in Russian shelling in Vovchansk, a border town that Russian forces are believed to partially occupy.

Ukraine's President Volodymyr Zelenskyy has asked the country's Western allies to provide it with additional air defense systems to protect Kharkiv.

Commenting on Telegram Sunday, Zelenskyy said "the world can stop Russian terror - and for this the lack of political will of the leaders must be overcome. Two "Patriots" [missile defense systems] will fundamentally change the situation for Kharkiv."

—️ Holly Ellyatt