Ukrainian President Volodymyr Zelensky announced Sunday that an initial group of roughly 100 citizens had been evacuated from the besieged Azovstal steel plant in the southern port city of Mariupol.

“Evacuation of civilians from Azovstal began. The 1st group of about 100 people is already heading to the controlled area. Tomorrow we’ll meet them in Zaporizhzhia,” he tweeted, referring to a city 220 kilometers (130 miles) to the northwest.

“Now they, together with #UN, are working on the evacuation of other civilians from the plant,” he said.

Earlier on Sunday, the United Nations had confirmed that a “safe passage operation is ongoing” at Azovstal which was being coordinated by the Red Cross and Russian and Ukrainian forces.

There were no further details due to safety concerns.

The vast Azovstal iron and steelworks is the last hold-out of Ukrainian forces in the devastated port city of Mariupol after a weeks-long onslaught by the Russian military.

Several hundred Ukrainian soldiers and civilians are sheltering in the maze of Soviet-era underground tunnels underneath the steelworks, many of whom require medical attention.

Stretching over 11 square kilometers (4.2 square miles), the complex is a sprawling warren of rail lines, warehouses, coal furnaces, factories, chimneys and tunnels seen as ideal for guerrilla warfare.