Russian and separatist forces in eastern Ukraine have reportedly taken full control of the eastern Ukrainian city of Lysychansk.

Russian state news agency Tass reported the news, quoting the defense ministry, on Sunday.

It comes after Russia earlier said it had encircled Ukraine's last major stronghold in the Luhansk region.

The defense ministry said its troops had captured the villages surrounding Lysychansk, encircling the area, and were fighting Ukrainian troops inside the town, Reuters reported.

“Russian troops and units of the Luhansk People’s Republic are fighting inside Lysychansk, completely defeating the encircled enemy,” the ministry said in a statement.

Ukraine's defense ministry has denied the claims, saying that the city was not under "full control" of Russia. But spokesperson Yuriy Sak added that if the entire Donbas region were to fall, it would not be “game over” for Ukraine.

It comes after an adviser to Volodymyr Zelenskiy said the loss of Ukraine's last large bastion in Luhansk was "indeed a threat". Oleksiy Arestovych added: “I do not rule out any one of a number of outcomes here. Things will become much more clear within a day or two.”

Former British army chief Lord Dannatt said “meaningful negotiations” could arise out of Russia potentially taking full control of Ukraine’s Luhansk and Donetsk provinces. His comments came before the Russian defense ministry reportedly claimed to have taken full control of Lysychansk, the last major Ukrainian stronghold in the region.

At least three people were killed and dozens of residential buildings damaged in the Russian city of Belgorod on Sunday, the region's governor said, after earlier reports of several blasts in the city near the Ukrainian border. Vyacheslav Gladkov said at least 11 apartment buildings and 39 private residential houses were damaged, including five houses destroyed. Reuters was not able to independently verify the reports and there was no immediate reaction from Ukraine. Gladkov said earlier on the Telegram messaging app: “Reasons for the incident are being investigated. Presumably, the air defense system worked.”