POKROVSK, Ukraine -- Concern mounted Saturday over Ukrainian fighters who became prisoners at the end of Russia's brutal three-month siege in Mariupol, as a Moscow-backed separatist leader vowed tribunals.
Russia claimed 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. Its seizure delivers Russian President Vladimir Putin a badly wanted victory in the war he began in February.
The Russian Defense Ministry released video of Ukrainian soldiers being detained after announcing that its forces had removed the last holdouts from the plant's miles of underground tunnels. Denis Pushilin, the pro-Kremlin head of an area of eastern Ukraine controlled by Moscow-backed separatists, claimed that 2,439 people were in custody. He said on Russian state TV that the figure includes some foreign nationals, though he did not provide further details.
Family members of the steel mill fighters, who authorities say 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. They are considered heroes by their fellow citizens.
Ukraine's Deputy Prime Minister Iryna Vereshchuk said Saturday that authorities "will fight for the return of every soldier" captured from Azovstal steelworks.
Convoys of buses, guarded by Russian armored vehicles, left the plant Friday. At least some Ukrainians were taken to a former penal colony, while Russian authorities said others were hospitalized.
Pushilin said the Ukrainians 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 plant's more than 2,400 defenders were members of the Azov Regiment, whose far-right origins have been seized on by the Kremlin as part of an effort to cast its invasion as a battle against Nazi influence in Ukraine.
The Ukrainian government has not commented on Russia's claim of capturing Azovstal, which for weeks remained Mariupol's last holdout of Ukrainian resistance. Ukraine's military had told the fighters that their mission was complete and they could come out. It described their extraction as an evacuation, not a mass surrender.
The end of the battle for Mariupol would help Putin offset some stinging setbacks, including the failure of Russian troops to take over Ukraine's capital, Kyiv, the sinking of the Russian Navy's flagship in the Black Sea and the continued resistance that has stalled an offensive in eastern Ukraine.
It also furthers Russia's quest to essentially create a land bridge from Russia stretching through the Donbas region to the Crimean Peninsula, which Moscow annexed 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 in the Black Sea region of Odesa as well as a significant cache of Western-supplied weapons in northern Ukraine's Zhytomyr region. There was no confirmation from the Ukrainian side.
In its morning operational report, the Ukrainian military general staff reported heavy fighting in much of eastern Ukraine, including the areas of Sievierodonetsk, Bakhmut and Avdiivka.
The Ukrainian military said that Russia is expected to relaunch an offensive on the strategically important city of Sloviansk, in the Donetsk region. The city 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.
Elsewhere in eastern Ukraine, the governor of the region of Luhansk, Serhii Haidai, said the only functioning hospital in the city of Sievierodonestsk had just three doctors and supplies for 10 days. In the Donetsk region, Russian shelling on Saturday killed seven civilians and injured 10 more, the regional governor said.
Ukrainian President Volodymyr Zelenskyy on Saturday dismissed the idea of launching a counterattack into Russian territory but emphasized that the Donbas region remains sovereign to Ukraine.
Speaking at a joint media conference with Portuguese Prime Minister Antonio Costa, Zelenskyy said his forces were fighting to "liberate our territory" and the price of "tens or hundreds of thousands of lives" was too high to surrender it. 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.
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.
Zelenskyy reiterated his intention to apply for European Union membership and accused Russia of blockading agricultural exports from Ukraine, which is known as the "breadbasket of Europe."
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), was 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.
Zelenskyy revealed in an interview published Friday that Ukrainian helicopter pilots braved Russian anti-aircraft fire to ferry in medicine, food and water to the steel mill as well as to retrieve bodies and rescue wounded fighters. A "very large" number of the pilots died on the missions, he said, calling them "absolutely heroic."
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.
McQuillan reported from Lviv. Stashevskyi reported from Kyiv. Associated Press journalists Yuras Karmanau in Lviv, Andrea Rosa in Kharkiv, Frances D'Emilio in Rome, and other AP staffers around the world contributed.
An aerial view of a residential area destroyed by Russian shelling in Irpin close to Kyiv, Ukraine, Saturday, May 21, 2022. (AP Photo/Efrem Lukatsky)
Portuguese Prime Minister Antonio Costa, center left, walks surrounded by media and security in Irpin, Ukraine, Saturday, May 21, 2022. (AP Photo/Efrem Lukatsky)
A Russian soldier patrols an area at the Kakhovka Hydroelectric Station, a run-of-river power plant on the Dnieper River in Kherson region, south Ukraine, Friday, May 20, 2022. The Kherson region has been under control of the Russian forces since the early days of the Russian military action in Ukraine. This photo was taken during a trip organized by the Russian Ministry of Defense. (AP Photo)
A woman with her child walks past a Russian soldier at an embankment of the Black Sea in Kherson, Kherson region, south Ukraine, Friday, May 20, 2022. The Kherson region has been under control of the Russian forces since the early days of the Russian military action in Ukraine. This photo was taken during a trip organized by the Russian Ministry of Defense. (AP Photo)
In this photo taken from video released by the Russian Defense Ministry on Saturday, May 21, 2022, Serhiy Volynskyy, nicknamed Volyna, commander of Ukraine's 36th Special Marine Brigade of Ukraine's Naval Forces lines up to be checked as he leaves the besieged Azovstal steel plant in Mariupol, in territory under the government of the Donetsk People's Republic, eastern Ukraine. (Russian Defense Ministry Press Service via AP)
In this photo taken from video released by the Russian Defense Ministry on Friday, May 20, 2022, Russian servicemen frisk Ukrainian servicemen after they leaved the besieged Azovstal steel plant in Mariupol, in territory under the government of the Donetsk People's Republic, eastern Ukraine. (Russian Defense Ministry Press Service via AP)
Debris cover the room of an apartment damaged during shelling in Kharkiv, eastern Ukraine, Saturday, May 21, 2022. (AP Photo/Bernat Armangue)
In this photo provided by the Ukrainian Presidential Press Office, Ukrainian President Volodymyr Zelenskyy, left, greets Portuguese Prime Minister Antonio Costa, during their meting in Kyiv, Ukraine, Saturday, May 21, 2022. (Ukrainian Presidential Press Office via AP)
Russian soldiers guard an area as a group of foreign journalists visit in Kherson, Kherson region, south Ukraine, Friday, May 20, 2022. The Kherson region has been under control of the Russian forces since the early days of the Russian military action in Ukraine. This photo was taken during a trip organized by the Russian Ministry of Defense. (AP Photo)