Ukraine war live updates: Ukraine and Russia exchange tit-for-tat strikes on border regions, leaving civilians dead and injured

This is CNBC's live blog tracking developments on the war in Ukraine. See below for the latest updates. 

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.

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