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WRAPUP 11-Battles rage at Ukrainian plant as U.N. rushes to evacuate civilians

(Adds Zelenskiy comments)

* Some 200 civilians still stuck in Mariupol steel plant

* Footage shows explosions striking plant, heavy smoke

* U.N. says new Mariupol evacuation operation underway

* Russia pledges safe passage for civilians in daytime

By Alessandra Prentice and Natalia Zinets

ZAPORIZHZHIA, Ukraine, May 5 (Reuters) - Scores of Ukrainians remained trapped on Thursday in a Mariupol steel works that has been rocked by heavy explosions as Russian forces fought for control of Ukraine's last stronghold in the ruined city and the United Nations rushed to evacuate civilians.

Ukrainian President Volodymyr Zelenskiy said more civilians had been rescued from Mariupol but Russia was shelling the Azovstal steel plant where an estimated 200 civilians were still in underground bunkers with little food or water.

President Vladimir Putin said Russia was prepared to provide safe passage for the civilians but reiterated calls for Ukrainian forces inside to disarm. Clinging on desperately, Ukrainian fighters have reported fierce battles with Russian troops.

Putin declared victory over Mariupol on April 21 and ordered his forces to seal off the Soviet-era plant but not venture inside its underground tunnel network. The Kremlin denies Ukrainian allegations that Russian troops stormed the plant in recent days.

Aerial footage of the plant, released Thursday by Ukraine's Azov Regiment, showed three explosions striking different parts of the vast complex, which was engulfed in heavy, dark smoke.

Reuters verified the footage location by matching buildings with satellite imagery, but was unable to determine when the video was filmed.

U.N. Secretary-General Antonio Guterres told the Security Council that a third operation was under way to evacuate civilians from Mariupol and Azovstal. In joint efforts with the Red Cross, the U.N. has helped nearly 500 civilians flee the area over the past week.

"We must continue to do all we can to get people out of these hellscapes," Guterres said. He declined to give details on the new operation "to avoid undermining possible success."

Zelenskiy said in a late-night video address that the U.N.-Red Cross evacuations in Mariupol continued on Thursday and that "people are on their way to safe territory."

He did not say if civilians were evacuated from Azovstal, but added: "Russian shelling and the storming of Azovstal are not stopping. But civilians still need to be taken out, women and children. A lot of children remain. Just imagine what sort of hell this is."