Russian troops have entered the outskirts of the Ukrainian city of Sievierodonetsk, the regional governor said on Monday, describing "very fierce" fighting in the ruins of a city that has become the focus of Moscow's offensive.

Russia has concentrated its firepower on the last major population centre still held by Ukrainian forces in the eastern Luhansk province, in a push to achieve one of President Vladimir Putin's stated objectives after three months of the war.

Incessant shelling has left Ukrainian forces defending ruins in...