The United Nations on Sunday confirmed that an operation to evacuate people from a steel plant in the bombed-out Ukrainian city of Mariupol is underway.

The UN humanitarian spokesperson Saviano Abreu told The Associated Press on Sunday that the effort to bring people out of Azovstal steel plant was being done in collaboration with the International Committee for the Red Cross and in coordination with Ukrainian and Russian officials.

Up to 1,000 civilians are believed to be at the plant with an estimated 2,000 Ukrainian fighters beneath the Soviet-era steel plant that is the only part of the city not occupied by the Russians. The plant has a network of underground tunnels.?

The UN spokesperson called the situation “very complex” and would not give further details, as per AP.