Concern is mounting over Ukrainian fighters who became Russia's prisoners after a brutal three-month siege in Mariupol.
A Russian-backed separatist leader vowed they would face tribunals.
Russian forces have claimed full control of the Azovstal steel plant, which was the final stronghold of Ukrainian resistance in Mariupol for weeks.
The Russian defence ministry released video footage of Ukrainian soldiers being detained after announcing that its forces had removed the last holdouts from the steel plant's miles of underground tunnels.
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 on Saturday that Ukraine "will fight for the return" of every one of them.
Among the plant's 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 seizure of the plant delivers Russian President Vladimir Putin a badly wanted victory in the war he began in February.
As western support rallies behind Ukraine, Poland's president Andrzej Duda arrived in Ukraine and will address the country's parliament, according to his office.
His country has taken in millions of Ukrainian refugees since Russia's invasion began.