After weeks of fighting, the strategically important city of Severodonetsk in eastern Ukraine is now completely in the hands of the Russian army, according to its mayor.

The city is "completely occupied" by the Russian armed forces, Mayor Oleksandr Strjuk said on Ukrainian television on Saturday.

On Friday, the Ukrainian army announced its withdrawal from the city of 100,000.

With the capture of Severodonetsk, the Russian army now controls by far the largest part of the Luhansk region.

The capture of the city is considered a strategically important step in order to conquer the entire Donbass, which includes Luhansk and the Donetsk region.

Parts of the economically important area in eastern Ukraine have been controlled by pro-Russian separatists since 2014.

Mayor Stryuk said civilians had begun evacuating the Azot chemical plant in Sievarodonetsk, where hundreds of residents of the city had fled from the Russian bombing raids.

“These people have spent almost three months of their lives in the cellars.

It's difficult mentally and physically," Strjuk said.

They now needed medication and psychological support.

At about the same time as the mayor's announcement, the pro-Russian separatists announced that their units and the Russian army had taken control of the chemical plant.

More than 800 civilians who had sought shelter there were "evacuated," separatist spokesman Ivan Filipenko told Telegram.

He gave no details as to where the people were being taken.

Earlier, another separatist representative, Andrei Maroshko, said Russian troops had now advanced to Lysychansk, the neighboring city of Sieverodonetsk.

There are street fights there.

Sevierodonetsk and Lysychansk are separated by a river and are around 80 kilometers east of Kramatorsk, the capital of the Ukrainian-controlled part of Donetsk region.

Should Russian troops also capture Lysychansk, they could then target Kramatorsk and Sloviansk, eventually capturing the entire Donbass region, Ukraine's industrial heartland.

A few weeks after the war began, the Kremlin issued the military goal of concentrating its forces on eastern Ukraine.

At the beginning of its war of aggression, Moscow declared that it wanted to overthrow the government in Kyiv.