Russia plans to seize the Azovstal steel plant in Mariupol, the last enclave of Ukrainian resistance in the port of strategically important city, by Monday, according to the Ukrainian government.
Oleksiy Arestovich, an adviser to Ukrainian President Volodymyr Zelensky, explained that Russian forces wanted the vast steel plant to fall into their hands on the 77th anniversary of Victory Day - the Soviet Union's successful resistance against Nazi Germany.
"Their 'ambition' is to 'take Azovstal and give Russian President Vladimir Putin 'victory' on May 9," Arestovich was quoted as saying by UNIAN news agency. "They really want it, but let's see if they can do it."
According to Ukrainian sources, Ukrainian troops, mostly members of the neo-Nazi Azov Battalion, as well as civilians are trapped in the facility. Efforts are being made to evacuate civilians with the help of the UN and the Red Cross.
A large military parade will be held in Moscow on Monday to mark the anniversary of the anti-fascist victory.
According to reports, a parade is expected to take place in Mariupol, which was visited this week by Sergei Kiriyenko, a top Kremlin official, according to Dennis Pushilin, leader of the pro-Russian separatists in the self-proclaimed Donetsk People's Republic.
Meanwhile, a third operation to pull civilians from Mariupol is due on Friday, the UN says, as more fighting was reported for control of the steelworks continues.
About 200 civilians are also believed to be hiding in bunkers in the plant.
UN chief Antonio Guterres said everything should be done to "get people out of these hellscapes".
Russia's Vladimir Putin said his forces are ready to give civilians safe passage, but fighters must surrender.
The Russian president has already declared victory in Mariupol, ordering his forces to seal off the sprawling industrial site - which was designed during the Cold War to serve as a nuclear bunker and has a network of tunnels deep below ground - rather than try to take control of it.
In his overnight address Ukraine's President Zelensky also said that Russian shelling and attempts to wrest control of the steelworks were ongoing.
"Just imagine this hell. And there are children! More than two months of constant shelling, bombing, constant death nearby," he said.
The Azov Battalion released drone footage showing explosions at the steelworks, but the date of the footage could not be verified. Earlier on Thursday Sviatoslav Palamar, a commander of the neo-Nazi outfit, said Ukrainian defenders were fighting "difficult, bloody battles" with Russian troops who had managed to enter part of the complex.
But the Kremlin denied its forces had tried to storm the plant and insisted that a humanitarian corridor was open as part of a three-day ceasefire that started on Thursday.
Mr Guterres said a third operation to evacuate civilians from the city was under way and would arrive in the city on Friday. Ukrainian Deputy Prime Minister Iryna Vereshchuk said on social media that this would happen at about midday.
At least 20 children are thought to be among the remaining civilians in the steelworks, and food and water supplies are dwindling fast.
On Sunday an initial group of civilians was evacuated from the plant. Footage showed mainly women and children being helped to walk over piles of rubble, and boarding a bus with missing windows.
Elsewhere, according to the BBC, the US provided intelligence that helped Ukraine sink the Russian warship Moskva, multiple US media outlets report.
Officials said Ukraine had asked the US about a ship in the Black Sea. The US identified it as the Moskva and helped confirm its location. Ukraine then targeted the ship
READ MORE: Russian TV shows “nuclear tsunami” wiping out UK.