A new attempt was underway to evacuate scores of civilians trapped in the heavily bombed steelworks in the city of Mariupol after bloody fighting with Russian forces thwarted efforts to bring them to safety the previous day, Ukraine said Friday. Meanwhile, Ukrainian fighters battling Russian forces in the tunnels beneath Mariupol’s immense steel plant refused to surrender in the face of relentless attacks.

Mariupol, a strategic southern port on the Azov Sea, has endured the most destructive siege of the 10-week-old war and the sprawling Soviet-era Azovstal steel plant is the last part of the city still in the hands of holdout Ukrainian fighters.

U.N.-brokered evacuations of some of the hundreds of civilians who had taken shelter in the plant's network of tunnels and bunkers began at the weekend, but were halted in recent days by renewed fighting.

"The next stage of rescuing our people from Azovstal is under way at the moment. Information about the results will be provided later," said Andriy Yermak, head of the Ukrainian presidential staff. He gave no more details.

Russia has turned its heaviest firepower on Ukraine's east and south, after failing to take the capital Kyiv in the early weeks following its Feb. 24 invasion. The new front is aimed at limiting Ukraine's access to the Black Sea, vital for its grain and metals exports, and linking Russian-controlled territory in the east to the Crimea Peninsula, seized by Moscow in 2014.

Moscow calls its actions a "special military operation" to disarm Ukraine and rid it of anti-Russian nationalism fomented by the West. Ukraine and the West say Russia launched an unprovoked war of aggression. More than 5 million Ukrainians have fled abroad since the start of the invasion.

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 II.

The battle, in the last Ukrainian stronghold of the strategic port city reduced to ruins by the Russian onslaught, appeared increasingly desperate on Thursday.

"They won’t surrender," Kateryna Prokopenko, wife of a commander who fought in the steel mill, said after speaking by phone to her husband, a leader of the steel plant defenders. "They only hope for a miracle."

She said her husband, Azov Regiment commander Denys Prokopenko, told her he would love her forever.

"I am going mad from this. It seemed like words of goodbye," she said.

The bloody battle came amid growing speculation that President Vladimir Putin wants to present the Russian people with a battlefield triumph – or announce an escalation of the war – in time for Victory Day on Monday. Victory Day is the biggest patriotic holiday on the Russian calendar, marking the Soviet Union’s triumph over Nazi Germany.

Some 2,000 Ukrainian fighters, by Russia’s most recent estimate, were holed up in a maze of tunnels and bunkers beneath Mariupol’s sprawling Azovstal steelworks. A few hundred civilians were also believed trapped there.

'Hospitals devastated'

Ukrainian President Volodymyr Zelenskyy said on Thursday that hundreds of hospitals and other medical institutions in the country had been devastated since the invasion, with many places in the south and east lacking even basic antibiotics.

"If you consider just medical infrastructure, as of today Russian troops have destroyed or damaged nearly 400 healthcare institutions: hospitals, maternity wards, outpatient clinics," Zelenskyy said in a video address to a medical charity group.

"This amounts to a complete lack of medication for cancer patients. It means extreme difficulties or a complete lack of insulin for diabetes. It is impossible to carry out surgery. It even means, quite simply, a lack of antibiotics."

The Kremlin says it only targets military or strategic sites and not civilians. Ukraine reports daily civilian casualties from Russian shelling and fighting, and accuses Russia of war crimes. Russia denies the allegations.

In Mariupol an estimated 200 civilians remained trapped underground in the Azovstal plant with little food or water.

Putin has said Russia is prepared to provide safe passage for the civilians but reiterated calls for Ukrainian forces inside to disarm.

Putin declared victory in Mariupol on April 21 and ordered his forces to seal off the plant but not venture inside its tunnel network.

The Kremlin denies Ukrainian allegations that Russian troops stormed the plant in recent days and said humanitarian corridors were in place. Russia's military promised to pause its activity for the next two days to allow civilians to leave.

Aerial footage of the plant, released on 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.