Russia claimed to have captured Mariupol Friday after a nearly three-month siege that reduced much of the strategic port city to ruins, with over 20,000 civilians feared dead, as Ukraine on the same day ordered its last troops holed up in Mariupol's Azovstal steelworks to lay down their arms, reports indicate.

There was no immediate confirmation from Ukraine.

Russian Defense Minister Sergei Shoigu reported to President Vladimir Putin the "complete liberation" of the Azovstal steelworks in Mariupol – the last stronghold of Ukrainian resistance – and the city as a whole, spokesperson Igor Konashenkov said.

Russia’s state news agency RIA Novosti quoted the ministry as saying a total of 2,439 Ukrainian fighters who had been holed up at the steelworks had surrendered since Monday, including over 500 on Friday.

A defense ministry video purporting to show the surrender showed a line of unarmed men approaching Russian soldiers outside the plant and giving their names. The Russians then carefully searched each man and their possessions and also appeared to be asking the defenders to show their tattoos

Ukraine's Azov regiment commander Denys Prokopenko had earlier said only the dead remained.

"The higher military command has given the order to save the lives of the soldiers of our garrison and to stop defending the city," he said in a video on Telegram.

"I now hope that soon, the families and all of Ukraine will be able to bury their fighters with honors."

Ukraine wants to exchange the surrendering Azovstal soldiers for Russian prisoners. But in Donetsk, the pro-Kremlin authorities are in turn threatening to put some of them on trial.

The International Committee of the Red Cross urged both sides to grant it access to prisoners of war and civilian internees, "wherever they are held."

"Many more families need answers," it said in a statement.

In Washington, Pentagon spokesperson John Kirby said all prisoners of war should "be treated in accordance with the Geneva Convention and the law of war."

Russian attacks on Mariupol, in southeast Ukraine, began shortly after the war started in late February.

As Russian troops gradually took control of the strategic city on the Sea of Azov through relentless aerial bombardments and a blockade, the last Ukrainian defenders in Mariupol took cover at the Azovstal site, which features an underground network of bunkers and tunnels. Water, food, medicine and ammunition were all in short supply.

In what amounted to an admission of defeat, Ukrainian President Volodymyr Zelenskyy declared the two-month-long battle for the last contested piece of Mariupol over at the start of the week, paving the way for the Ukrainian troops to be taken prisoner by the Russians.

On Monday, the first 264 soldiers surrendered, including more than 50 seriously injured. According to Russia, more were captured on Thursday, but commanders and some fighters had continued to hold their ground.

But earlier on Friday, the remaining Ukrainian defenders of the steel plant had decided that they saw no point in continuing on, according to a video message released by Prokopenko, commander of the Azov Regiment.

"The army leadership has given the order to stop defending the city," Prokopenko said, adding the decision was taken to protect the lives and health of the soldiers.

A total of 2,439 Ukrainian combatants have been taken into Russian captivity at the steelworks since May 16, according to the ministry.

Moscow had always said they it assumed there were about 2,500 Ukrainian fighters at the site. The government in Kyiv, on the other hand, had given their number as only 1,000.

The Kremlin has not clarified what comes next. Kyiv has suggested they could be exchanged for Russians held by Ukraine, although some in Moscow have called for the Ukrainian forces to be treated as criminals and put on trial.

Most of the devastated city's prewar population of 400,000 has fled in recent weeks.

Russia was focused on Mariupol because conquering the city would help create a corridor between the Crimean peninsula and pro-Russian separatist regions in eastern Ukraine.

Moscow claimed success in that part of Ukraine, too.

Defence Minister Shoigu said earlier in the day that Russian troops were continuing their advance across the eastern Donbass region and the "liberation" of the Luhansk People's Republic was close at hand,

The Donbass, where fighting has intensified in recent weeks, contains the two self-declared "people's republics" of Donetsk and Luhansk, which were formed by pro-Russian separatists in 2014.

Putin had announced that Russia would recognize the breakaway regions as independent of Ukraine shortly before launching his invasion on Feb. 24.

Meanwhile, the Organization for Security and Cooperation in Europe (OSCE) said expanding its investigation into human rights violations committed during Russia's war.

Observers have been sent to Ukraine to interview witnesses and survivors, the OSCE Office for Democracy and Human Rights says.

The office has also interviewed people who fled to neighboring countries before Moscow's invasion began.

The OSCE is focusing on the civilian population and prisoners of war, with attacks on civilians, killings, abductions and torture the most urgent issues. It will likely be months before a report is ready.

Only a small number of the alleged atrocities against civilians in Ukraine for which Russian soldiers stand accused are even reported, according to Ukrainian officials.

"We have found evidence of many crimes, including sexual crimes, which often go unreported," Ukrainian Deputy Prime Minister Olha Stefanishyna told the Italian newspaper La Repubblica. She said it was a Russian tactic to break the morale of the Ukrainian population.