Ukrainian president and the head of the US Central Intelligence Agency have warned that the Kremlin may use tactical nuclear weapons during the conflict.

On Thursday, Russian troops fired on two minibuses evacuating civilians in the Kharkiv region, killing seven people and injuring 34, according to officials in Kharkiv’s second city.

On Friday, a “cluster munition” hit a residential area in Mykolaiv, killing five people and injuring 15.

Russian forces are repositioning to launch a major assault on the eastern Donbas region, Ukraine and its western allies say.

Despite a month-long siege and massive bombardment that Ukrainian officials believe killed 20,000 civilians, Russia claims control of the strategic Azov Sea port of Mariupol.

“Mariupol is in a bad situation. Now there’s fighting. The Russian army keeps calling in reinforcements to storm the city. As of now, the Russians haven’t completely captured it,” said Oleksandr Motuzyanyk of the Ukrainian defence ministry.