ZAPORIZHZHIA, Ukraine (AP) — 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 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 Soviet-era steel plant that is the only part of the city not occupied by the Russians.

The United Nations labored Sunday to get civilians out of the bombed-out ruins of the Ukrainian city of Mariupol, while US House Speaker Nancy Pelosi revealed she visited Ukraine’s president to show unflinching American support for the country’s defense against Russian aggression.

Russia’s high-stakes offensive in coastal southern Ukraine and the country’s eastern industrial heartland has Ukrainian forces fighting village by village and more civilians fleeing airstrikes and artillery shelling as war draws near their doorsteps.

As many as 100,000 people are believed to still be in blockaded Mariupol, including up to 1,000 civilians who were hunkered down with an estimated 2,000 Ukrainian fighters beneath a sprawling Soviet-era steel plant that is the only part of the city not occupied by the Russians.