At least some were taken by the Russians to a former penal colony in territory controlled by Moscow-backed separatists. A separatist official said others were hospitalised.
It was not clear how many fighters were left in the Azovstal steelworks, but Russia most recent estimate came in at around 2,000 troops.
It comes after a Ukrainian general said the evacuation of troops from the razed port city was continuing on Thursday, without providing any detail.
“In the Mariupol direction, measures are being taken to evacuate our heroes,” deputy chief of the main operational department of the General Staff of Ukraine’s Armed Forces Oleksiy Gromov said.
His remarks come amid international fears the Kremlin will take reprisals against the prisoners in Mariupol’s bombed-out steel plant.
The International Committee of the Red Cross (ICRC) registered hundreds of the soldiers as prisoners of war in a step toward ensuring their humane treatment under the Geneva Conventions.
On Tuesday, the NGO began registering the Ukrainian troops as they started to leave the site. The process has been ongoing since.
The registration process involves filling out a form with personal details like name, date of birth, and next of kin. This information allows the ICRC to track POWs and help them keep in touch with their families.
In accordance with the mandate given to the ICRC by the states parties to the 1949 Geneva Conventions, the ICRC must have immediate access to all POWs in all places where they are held.
Also, the ICRC must be allowed to interview POWs without witnesses, and the duration and frequency of these visits should not be unduly restricted.
Amnesty International has been pushing for ICRC staff tonbe given access to the troops as a result of what if referred to as lawless executions allegedly carried out by Russian forces in Ukraine. The Azovstal defenders “must not meet the same fate,” it said.
If Russia were to capture the vast plant it would allow its forces to claim complete control of Mariupol.
Elsewhere, officials in the Russian border region of Belgorod have claimed that the area has been shelled from the direction of Ukraine over the past week.
The region’s governor Viacheslav Gladkov said that the village Solokhi was shelled “from the Ukrainian side” several times over a period of days. On Telegram on Wednesday, he said one man was wounded.
Earlier this week, on Tuesday, Mr Gladkov reported that Belgorod village Bezimeno was also shelled “from the Ukrainian side” and that there was one person who sustained “slight injury”.
On 13 May, he said an 18-year-old man – Ruslan Nefedov – was buried after he “died during the shelling of our border village of Solokhi by the Armed Forces of Ukraine.”
The first alleged Ukrainian attack on Belgorod amid Russia’s invasion of Ukraine was reported on 1 April, when two helicopters struck an oil depot.