The interview marks the first time the Kremlin has commented on the cases of Alexander Drueke and Andy Huynh, both US military veterans, according to NBC.

"They're soldiers of fortune and they were involved in illegal activities on the territory of Ukraine. They were involved in firing and shelling our military personnel. They were endangering their lives," Peskov told the network in English.

"They should be held responsible for those crimes that they have committed," he added in the first bits of the interview made public.

"Those crimes have to be investigated."

When pressed on what crimes the Americans had committed, Peskov admitted their specific offenses were not yet known but claimed that they would not be covered by the Geneva conventions on prisoners of war.

"They are not (in the) Ukrainian army, so they are not subject to the Geneva Conventions," the Kremlin spokesman said.

Last week, a Russian state TV channel aired videos on social media of Drueke and Huynh.

The circumstances under which the two men were being held were not fully clear, nor who specifically was holding them. Peskov would only say they were being held by "authorities."

As to whether they could face the death penalty, Peskov said: "It depends on the investigation."

US President Joe Biden said Friday that he did not know the whereabouts of Drueke and Huynh, who are believed to have joined other foreigners in volunteering alongside Ukrainian troops.

A US State Department spokesperson 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 spokesperson told AFP.

Rolling Stone magazine also reported Monday that a second American citizen had died while fighting in Ukraine after stepping on a landmine.

-- Kremlin says WNBA star not a 'hostage' --

An obituary for 52-year-old Stephen Zabielski, posted in early June to a Florida newspaper's website, said that he had died on May 15 in Ukraine's eastern Zaporizhia region.

A 22-year-old former marine, Willy Joseph Cancel, was confirmed as the first American killed fighting for Ukraine in late April.

On the case of WNBA superstar Brittney Griner, who is being held in pre-trial detention in Russia on drug charges, Peskow strongly rejected that she was being held as a hostage.

"We cannot call her a hostage. Why should we call her a hostage? She violated Russian law and now she is being prosecuted," the Kremlin spokesman said.

Griner, a two-time Olympic gold medallist and WNBA champion, was detained at Moscow airport in February on charges of carrying vape cartridges with cannabis oil in her luggage, which could carry a 10-year prison sentence.

Washington says that Russia has "wrongfully detained" the 31-year-old Griner and turned her case over to the US special envoy in charge of hostages.

Her pre-trial detention has been extended until July 2.