Fierce fighting continues in the eastern Ukrainian region of Donetsk, as Russia's military appears to be focusing its assault on Avdiivka, a regional stronghold for Ukrainian forces.

Russia's offensive in Avdiivka has intensified since the beginning of October.

A US think tank, the Institute for the Study of War, said in its analysis released on Sunday that Russia "currently has 40,000 personnel in the Avdiivka direction and is transferring additional manpower to the area."

Russia is said to have suffered heavy casualties in the fight around Avdiivka. Ukraine's military said on Monday that six Russian warplanes, including a Sukhoi fighter jet, were shot down.

Ukraine's counteroffensive continues in the south. In the region of Kherson, Ukrainian troops have likely crossed to the Russian-occupied eastern side of the Dnipro River.

The Russian military has reportedly dismissed the head of a battlegroup in the Kherson area. Russian media reported on Sunday that Colonel-General Mikhail Teplinsky, commander of the Russian airborne force, had been newly appointed.

Russia's defense ministry has yet to make an official announcement. The Institute for the Study of War said the reported dismissal may be a sign that the Kremlin or the Russian military command was worried about the predecessor's ability to repel Ukraine ground operations in the region.