KYIV, May 1 - A long-awaited effort to evacuate people from a steel plant in the Ukrainian city of Mariupol was underway Sunday, the United Nations said, while U.S. House Speaker Nancy Pelosi revealed she visited Ukraine’s president to show unflinching American support for the country’s defense against Russian aggression.

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

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 the Soviet-era steel plant - the only part of the city not occupied by the Russians.

Abreu called the situation “very complex” and would not give further details. Like other evacuations, success of the mission in Mariupol depended on Russia and its forces in a long series of checkpoints before reaching Ukrainian ones.

People who have fled Russian-occupied areas have at times described their vehicles being fired on. Ukrainian officials have repeatedly accused Russian forces of shelling evacuation routes on which the two sides had agreed.

Pelosi, a California Democrat who is second in line to succeed the president, is the most senior American lawmaker to travel to Ukraine since Russia invaded Ukraine on Feb. 24. Her Saturday visit came just days after Russia launched rockets at the capital during a visit by U.N. Secretary-General António Guterres.

During a Sunday news conference in the Polish city of Rzeszow, Pelosi said she and other members of a U.S. congressional delegation met with Ukrainian President Volodymyr Zelenskyy in Ukraine’s capital, Kyiv, for three hours and brought him “a message of appreciation from the American people for his leadership.”

“All the leaders of the free world know what Russia has done to Mariupol. And Russia will not go unpunished for this,” Zelenskyy said in his nightly video address. He warned that Russia was “gathering additional forces for new attacks against our military in the east of the country.”

Limited evacuations from the city took place on Saturday, but the details had been unclear given the number of parties involved in the negotiations and the volatile situation on the ground.

The Russian Defense Ministry said a total of 46 people, a group of 25 and another numbering 21, were evacuated from areas near the Azovstal plant.

A top official with the Azov Regiment, the Ukrainian unit defending the steelworks, said Saturday that 20 women and children were evacuated from the plant itself. Civilians have sheltered in a maze of underground tunnels while the plant has been under siege.

In a video posted on the regiment’s Telegram channel, regiment Deputy Commander Sviatoslav Palamar called for the evacuation of wounded Ukrainian fighters as well as civilians. “We don’t know why they are not taken away and their evacuation to the territory controlled by Ukraine is not being discussed,” he said. (ap/ez)