Ukrainian forces are stepping up their counter-offensive in southern regions, including Kherson, which Russia claims is under its control.

The US-based think tank Institute for the Study of War said on Saturday, "Ukrainian forces seek to disrupt key logistics nodes that support the Russian operations in the south and chip away at Russian military capabilities."

The institute cited a Ukrainian presidential adviser who said in a media interview that Ukrainian troops are systematically destroying Russia's operational logistical supply system with artillery and precision weapon strikes.

Russian forces are continuing their attacks in eastern and southern Ukraine.

The governor of the eastern region of Donetsk said on social media that four civilians were killed in Russian attacks on Saturday.

The governor of the southern region of Mykolaiv said on Sunday that a massive missile strike late at night caused damage to facilities, including medical and educational institutions.

Russia began a major military exercise known as Vostok in its Far East region on Thursday.

The Russian defense ministry on Sunday released footage of the training involving multiple-launch rocket systems in the Sakhalin region.

Observers say Russia apparently hopes to show through the drills that it has sufficient military power.