Soldiers captured in wartime are protected by international agreements, but Russia has said it will treat fighters from Ukraine's Azov regiment -- an ultra-nationalist unit the Kremlin calls "neo-Nazi" -- as "terrorists" and try them for war crimes.
Several NGOs have condemned Ukraine, meanwhile, for violating the rights of Russian prisoners of war after videos emerged of humbled Russian servicemen regretting their actions.
AFP looks at the key questions surrounding the fate of prisoners of war in the Ukraine conflict.
- How many are there? -
As in all conflicts, information from the ground is often fragmentary and difficult to verify independently, and prisoner of war figures are no exception.
Russian Defence Minister Sergei Shoigu said 3,826 Ukrainian prisoners were taken in the port city of Mariupol, including 2,439 during the surrender of the Azovstal steelworks, with 1,387 marines captured beforehand.
The International Committee of the Red Cross has said it recorded "hundreds of Ukrainian prisoners of war" in Azovstal.
Moscow's envoy to the separatist region of Lugansk gave a figure Thursday of 8,000 Ukrainian prisoners for eastern Ukraine's two breakaway regions of Lugansk and Donetsk, according to the TASS news agency.
Hundreds more join them every day, Rodion Miroshnik was quoted as saying.
Ukraine has provided no prisoner of war figures despite repeated requests from AFP.
- What is their status? -
Regular army soldiers who fall into enemy hands are considered prisoners of war as defined by the Third Geneva Convention of 1949, which also applies when war has not been officially declared.
"The POW status applies to the members of the armed forces or to militiamen that are part of the armed forces, who are captured during a war," said William Schabas, a professor of international law at London's Middlesex University.
These prisoners have rights and should be protected against insults, public displays and all acts of violence or intimidation, he said.
But NGOs claim that some of these rights have been flouted since the beginning of the conflict.
In March, Human Rights Watch (HRW) asked Ukraine to stop parading repentant Russian prisoners of war for the media, an action also criticised by the Red Cross.
HRW has also called on the Ukrainian authorities to investigate potential war crimes against Russian prisoners, after footage emerged of Ukrainian soldiers appearing to shoot some of them in the legs.
Amnesty International has condemned the treatment meted out to Ukrainian prisoners from Azovstal, presented in Russian media "in a dehumanising way" as "neo-Nazis."
Julia Grignon, a researcher at France's Institute for Strategic Research at the Ecole Militaire war college (IRSEM), said tracking by the Red Cross would play a key role because perpetrators can be held accountable at a later date.
- What about prisoner exchanges? -
Now a common practice, prisoner exchanges are not governed by international law and take the form of negotiated agreements.
Several exchanges of soldiers and civilians between Ukraine and Russia have taken place since the start of the invasion, without being systematically confirmed by the two sides.
Kyiv's request to exchange an oligarch ally of Russian President Vladimir Putin, Viktor Medvedchuk, for Ukrainian prisoners remains under consideration by Moscow, Russia's negotiator Leonid Slutsky said Sunday.
"There might be exchanges proportionate to the importance given to certain people. Gilad Shalit, an Israeli prisoner, was so symbolic that the Israelis freed 1,027 Palestinians to obtain his release" in 2011, Grignon said.
- Can they be tried? -
Prisoners of war cannot be put on trial simply for having taken part in the fighting, but soldiers accused of crimes can be prosecuted, Grignon said.
The first Russian soldier to face trial for war crimes in Ukraine since Moscow's invasion was sentenced to life imprisonment in Kyiv on Monday for murdering a civilian.
Russian authorities, meanwhile, have indicated they will put Azov regiment fighters on trial as "Nazi criminals."
Grignon said such a procedure would be incompatible with humanitarian law. "You cannot label them Nazis or terrorists, you have to prosecute them for acts they are suspected of having committed," she said.
"It is clear to everyone that the Azov fighters are members of the Ukrainian armed forces, it seems that they have militiamen who are associated with the armed forces, then they should be treated as prisoners of war as well," added Schabas.
Members of the private Russian security company Wagner, with which Moscow denies any links, can be deemed prisoners of war if they are captured while fighting alongside Russia's army.
If they are not, they can be treated as civilians taking part in hostilities and are not entitled to the prisoner of war status.