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.

The first group of 100 refugees were being led away from the Azovstal steel site Sunday by Red Cross monitors, Ukrainian President Volodymyr Zelenskyy’s office said.

Reporters for the Reuters news agency reported a second group of 60 Mariupol residents were seen arriving by bus at a village in the Donetsk region away from the front lines.

“The 1st group of about 100 people is already heading to the controlled area,” Mr. Zelenskyy said on Twitter. “Tomorrow we’ll meet them in Zaporizhzhia. Grateful to our team! Now they, together with [the U.N.], are working on the evacuation of other civilians from the plant.”