By Alessandra Prentice and Natalia Zinets
ZAPORIZHZHIA, Ukraine (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."
HUMANITARIAN CORRIDORS
Ukraine's stubborn defence of Azovstal has underlined Russia's failure to take major cities in a 10-week-old war that has united Western powers in arming Kyiv and punishing Moscow with sanctions.
Russia's military promised to pause its activity in Azovstal during the day on Thursday and the next two days to allow civilians to leave, after fighting prevented evacuations from the plant on Wednesday. The Kremlin said humanitarian corridors from the plant were in place.
A Ukrainian fighter who said he was holed up in Azovstal accused Russian forces of breaching the plant's defences for a third day despite an earlier pledge by Moscow to pause military activity to permit civilian evacuations.
"Heavy, bloody fighting is going on," said Captain Sviatoslav Palamar of Ukraine's Azov Regiment. "Yet again, the Russians have not kept the promise of a ceasefire." Reuters could not independently verify his account or location.
Zelenskiy says a truce is needed to allow rescue workers to clear people trapped under rubble in the plant. "We are waiting for an effective ceasefire regime," he said.
Mariupol is an important target in Russian efforts to cut Ukraine off from its coastal grain and metals export routes, as well as to link Russian-controlled territory in the east of the country to Crimea, seized by Moscow in 2014.
One women who fled Mariupol last month with her sister said she breaks down in tears as she thinks of her mother, whom they left behind and have since lost contact with.
"When I last saw my mother, it felt like a normal visit in unusual circumstances," said Nicole, who declined to give her family name. "I could never have imagined that I wouldn't be able to call and ask them how they are doing."
MORE SANCTIONS LOOM
Sweeping sanctions from Washington and European allies have hobbled Russia's $1.8 trillion economy, while billions of dollars worth of military aid has helped Ukraine frustrate the invasion.
European Union countries are "almost there" in agreeing the bloc's proposed new package of sanctions against Russia, including an oil embargo, EU foreign policy chief Josep Borrell said.
The Kremlin said Russia was weighing responses to the EU plan.
Ukraine and Russia said fighting had been heavy across the south and east over the past day.
Ukrainian authorities reported shelling of towns near a frontline that divides territory it holds in the Donetsk and Luhansk regions from land held by Russian-backed separatists.
The Ukrainian air force said it had downed three Russian cruise missiles and four aircraft, including two Sukhoi fighter jets, while Russia said it had killed 600 Ukrainian soldiers overnight. Reuters could not independently verify either report.
Ukrainian officials have warned that Russia might step up its offensive before May 9, when Moscow commemorates the Soviet Union's victory over Nazi Germany in World War Two.
Russia calls its actions a "special military operation" to disarm Ukraine and protect it from fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.
More than 5 million Ukrainians have fled abroad since the start of the invasion.