Russian forces fought their way into the centre of Severodonetsk from the north and south despite fierce resistance on Monday as ground troops attempted to seize the strategic city in eastern Ukraine.

The Ukrainian military also said that Russian forces were reinforcing their positions on the city’s outskirts and bringing additional equipment and ammunition into the area to press their offensive.

"Unfortunately we have disappointing news, the enemy is moving into the city,” Luhansk Governor Serhiy Haidai told national television. "The fighting continues, the situation is very difficult.”

Severodonetsk – with a prewar population of about 100,000 people – is one of several important urban hubs that lie on Russia’s path to capturing the entire Luhansk region, a key objective of Moscow’s military.

"Russian troops have entered the city and street fighting is going on,” Severodonetsk Mayor Oleksandr Striuk told The Associated Press, adding Ukrainian defenders were fighting fiercely to push the Russians out.

"The Russian troops have advanced a few blocks toward the city centre. We have no power and no communications. The city has been completely ruined,” said Striuk.

About 13,000 civilians left in the city are sheltering in basements and bunkers to escape relentless Russian bombardment. "The number of victims is rising every hour, but we are unable to count the dead and the wounded amid the street fighting,” Striuk said.

About 1,500 residents of the city have died since the start of the war, he said.

Severodonetsk, the largest city that Kyiv still partly controls in the Luhansk region of the Donbas, has been the focus of Russia’s attacks in recent weeks.