Two Americans captured in Ukraine while fighting with Kyiv's military were "endangering" Russian soldiers and should be "held accountable for those crimes", Kremlin spokesman Dmitry Peskov said on Monday.
Mr Peskov's remarks to NBC News were the first time the Kremlin has commented on the cases of Alexander Drueke and Andy Huynh, both US military veterans, the network said.
"They're soldiers of fortune and they were involved in illegal activities on the territory of Ukraine," Mr Peskov said.
"They were involved in firing and shelling our military personnel. They were endangering their lives.
"They should be held responsible for those crimes that they have committed. Those crimes have to be investigated."
Mr Peskov admitted that their specific offences were not yet known, but he claimed that they would not be covered by the Geneva Conventions on prisoners of war.
"They are not Ukrainian army, so they are not subject to the Geneva Conventions," he said.
Last week, a Russian state-TV channel aired videos on social media of Mr Drueke and Mr Huynh.
The circumstances under which the two men were being held were not fully clear, nor was who was holding them. Mr Peskov would only say they were being held by "authorities."
As to whether they could face the death penalty, he said: "It depends on the investigation."
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US President Joe Biden said Friday that he did not know the whereabouts of Mr Drueke and Mr Huynh, who are believed to have joined other foreigners in volunteering alongside Ukrainian troops.
A US State Department representative on Saturday confirmed American authorities had seen the photos and videos of the two US citizens "reportedly captured by Russia's military forces in Ukraine".
"We are closely monitoring the situation and our hearts go out to their families during this difficult time," the representative told AFP.
On the case of WNBA superstar Brittney Griner, who is being held in pre-trial detention in Russia on drug charges, Mr Peskov strongly rejected the idea that she was being held as a hostage.
"We cannot call her a hostage," he said. "Why should we call her a hostage? She violated Russian law and now she is being prosecuted."
Griner, 31, a two-time Olympic gold medallist and WNBA champion, was detained at Moscow airport in February on charges of carrying in her luggage vape cartridges with cannabis oil, which could carry a 10-year prison sentence.
Washington says that Russia has "wrongfully detained" her and turned her case over to the US special envoy in charge of hostages.
Her pre-trial detention has been extended until July 2.