Civilians trapped in the grim battle for control of the Ukrainian port city of Mariupol have been allowed to start leaving the city by besieging Russian forces, the United Nations and the Ukrainian government said Sunday.

Invading Russian troops have been trying to oust a stubborn detachment of Ukrainian troops at a vast steel mill complex on the edge of the city, with about a thousand Ukrainian civilians also trapped at the site.

Previous efforts to establish a “humanitarian corridor” to bus out the civilians have repeatedly broken down.

Ukrainian authorities are planning to evacuate more civilians from Mariupol on Monday, after dozens were finally brought to safety following weeks trapped under heavy fire in the strategic port city’s Azovstal steel complex.

The civilians had been sheltering in bunkers beneath the steelworks that is the last redoubt for Ukrainian forces in Mariupol.

On Sunday, Ukrainian president Volodymyr Zelenskiy said about 100 civilians evacuated from the steel works and would arrive in the Ukrainian-controlled city of Zaporizhzhia on Monday morning.

Russian forces shelled the Azovstal steel plant soon after some civilians were evacuated from the besieged facility, Ukrainian military officials said ahead of fresh evacuations planned for Ukraine's final stronghold in Mariupol on Monday.