ZAPORIZHZHIA, UKRAINE, MAY 01

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

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

He called the situation 'very complex' and would not give further details.

Up to 1,000 civilians are believed to be hunkered down with an estimated 2,000 Ukrainian fighters beneath the sprawling Soviet-era steel plant that is the only part of the city not occupied by the Russians.