Ukraine's military said on Monday that its forces drove back Russian attacks around the heavily fortified city of Avdiivka in the eastern region of Donetsk. Russia is reportedly continuing its offensive there.

US think tank the Institute for the Study of War said in its assessment that Russian forces continued operations to encircle Avdiivka on October 15 "but have yet to make further gains amid a likely decreasing tempo of Russian operations in the area."

It added that they will remain a threat to Ukrainian forces in the area.

Russia's Black Sea Fleet was hit by Ukrainian attacks last month, including missile strikes on its headquarters and other facilities in Crimea.

Britain's defense ministry said on Saturday that after a series of strikes in August and September, the Russian Fleet has "likely doubled down on its defensive and reactive posture."

The ministry also noted that the country has relocated many of its prestige naval assets to eastern areas, such as the Novorossiysk naval base in southern Russia.

The Ukrainian forces said on Monday that Russia increased the presence of missile carrier vessels in the Back Sea, and that in addition to a submarine, two more frigates are on combat duty.