With Russia claiming to have taken prisoner nearly 2,500 Ukrainian fighters from the besieged Mariupol steel plant, concerns grew about their fate as a Moscow-backed separatist leader vowed they would face tribunals.

Russia has declared its full control of the Azovstal steel plant, which for weeks was the last holdout in Mariupol and a symbol of Ukrainian tenacity in the strategic port city, now in ruins with more than 20,000 residents feared dead. The seizure gives Russian President Vladimir Putin a badly wanted victory in the war he began nearly three months ago.

As the West rallies behind Ukraine, Polish President Andrzej Duda arrived in Ukraine on an unannounced visit and will address the country's parliament on Sunday, his office said.

Poland, which has welcomed millions of Ukrainian refugees since the start of the war, is a strong supporter of Ukraine's desire to join the European Union. With Russia blocking Ukraine's sea ports, Poland has become a major gateway for Western humanitarian aid and weapons going into Ukraine and has been helping Ukraine get its grain and other agricultural products to world markets.

The Russian Defense Ministry released video of Ukrainian soldiers being detained after announcing that its forces had removed the last holdouts from the Mariupol plant's extensive underground tunnels. It said a total of 2,439 had surrendered.

Family members of the fighters, who came from a variety of military and law enforcement units, have pleaded for them to be given rights as prisoners of war and eventually returned to Ukraine. Deputy Prime Minister Iryna Vereshchuk said Saturday that Ukraine "will fight for the return" of every one of them.

Denis Pushilin, the pro-Kremlin head of an area of eastern Ukraine controlled by Moscow-backed separatists, said the captured fighters included some foreign nationals, though he did not provide details. He said they were sure to face a tribunal. Russian officials and state media have sought to characterize the fighters as neo-Nazis and criminals.

"I believe that justice must be restored. There is a request for this from ordinary people, society, and, probably, the sane part of the world community," Russian state news agency Tass quoted Pushilin as saying.

Among the defenders were members of the Azov Regiment, whose far-right origins have been seized on by the Kremlin as part of its effort to cast the invasion as a battle against Nazi influence in Ukraine.

A prominent member of Russia's parliament, Leonid Slutsky, said Moscow was studying the possibility of exchanging the Azovstal fighters for Viktor Medvedchuk, a wealthy Ukrainian with close ties to Putin who faces criminal charges in Ukraine, the Russian news agency Interfax reported. Slutsky later walked back those remarks, saying he agreed with Pushilin that their fate should be decided by a tribunal.

The Ukrainian government has not commented on Russia's claim of capturing Azovstal. Ukraine's military had told the fighters their mission was complete and they could come out. It described their extraction as an evacuation, not a mass surrender.

The capture of Mariupol furthers Russia's quest to create a land bridge from Russia stretching through the Donbas region to the Crimean Peninsula, which Moscow seized from Ukraine in 2014.

The impact on the broader war remained unclear. Many Russian troops already had been redeployed from Mariupol to elsewhere in the conflict.

Russian Defense Ministry spokesman Igor Konashenkov reported Saturday that Russia destroyed a Ukrainian special-operations base near Odesa, Ukraine's main Black Sea port, as well as a significant cache of Western-supplied weapons in northern Ukraine's Zhytomyr region. There was no confirmation from the Ukrainian side.

The Ukrainian military reported heavy fighting in much of the Donbas in eastern Ukraine.

"The situation in Donbas is extremely difficult," President Volodymyr Zelenskyy said in his nightly video address to the nation. "As in previous days, the Russian army is trying to attack Sloviansk and Sievierodonetsk." He said Ukrainian forces are holding off the offensive "every day."

Sievierodonetsk is the main city under Ukrainian control in the Luhansk region, which together with the Donetsk region makes up the Donbas. Gov. Serhii Haidai said the only functioning hospital in the city has just three doctors and supplies for 10 days.

Sloviansk, in the Donetsk region, is critical to Russia's objective of capturing all of eastern Ukraine and saw fierce fighting last month after Moscow's troops backed off from Kyiv. Russian shelling on Saturday killed seven civilians and injured 10 more elsewhere in the region, the governor said.

A monastery in the Donetsk region village of Bohorodichne was evacuated after being hit by a Russian airstrike, the regional police said Saturday. About 100 monks, nuns and children had been seeking safe shelter in the basement of the church and no one was hurt, the police said in a Facebook post, which included a video showing extensive damage to the monastery as well as nuns, monks and children boarding vans on Friday for the evacuation.

Zelenskyy on Saturday emphasized that the Donbas remains part of Ukraine and his forces were fighting to liberate it.

Speaking at a joint news conference with Portuguese Prime Minister Antonio Costa, he pressed Western countries for multiple-launch rocket systems, which he said "just stand still" in other countries yet are key to Ukraine's success.

Portugal and Poland, where Costa stopped for talks before traveling on to Kyiv, support bringing Ukraine into the European Union quickly, even if some other EU members balk at granting it speedy access.

U.S. President Joe Biden signed off Saturday on a fresh, $40 billion infusion of aid for Ukraine, with half for military assistance. Portugal pledged up to 250 million euros, as well as continued shipments of military equipment.

Mariupol, which is part of the Donbas, was blockaded early in the war and became a frightening example to people elsewhere in the country of the hunger, terror and death they might face if the Russians surrounded their communities.

The seaside steelworks, occupying some 11 square kilometers (4 square miles), were a battleground for weeks. Drawing Russian airstrikes, artillery and tank fire, the dwindling group of outgunned Ukrainian fighters held out with the help of airdrops that Zelenskyy said cost the lives of many "absolutely heroic" helicopter pilots.

The Russian Defense Ministry on Saturday released video of Russian troops taking into custody Serhiy Volynskyy, the commander of the Ukrainian Navy's 36th Special Marine Brigade, which was one of the main forces defending the steel plant. The Associated Press has not been able to independently verify the date, location and conditions of the video.

With Russia controlling the city, Ukrainian authorities are likely to face delays in documenting evidence of alleged Russian atrocities in Mariupol, including the bombings of a maternity hospital and a theater where hundreds of civilians had taken cover. Satellite images in April showed what appeared to be mass graves just outside Mariupol, where local officials accused Russia of concealing the slaughter by burying up to 9,000 civilians.

An estimated 100,000 of the 450,000 people who resided in Mariupol before the war remain. Many, trapped by Russia's siege, were left without food, water and electricity.

The Ukrainian mayor of Mariupol warned Saturday the city is facing a health and sanitation "catastrophe" from mass burials in shallow pits across the ruined city as well as the breakdown of sewage systems. Vadim Boychenko said summer rains threaten to contaminate water sources as he pressed Russian forces to allow residents to safely leave the city.

"In addition to the humanitarian catastrophe created by the (Russian) occupiers and collaborators, the city is on the verge of an outbreak of infectious diseases," he said on the messaging app Telegram.

From The Associated Press