Two Americans apprehended in Ukraine while fighting alongside Kiev's troops were "endangering" Russian servicemen and should be "held accountable for those crimes," Kremlin Spokesperson Dmitry Peskov told NBC News on Monday.

According to NBC, the interview is the first time the Kremlin has spoken on the cases of Alexander Drueke and Andy Huynh, both US military veterans.

"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 news outlet.

"They should be held responsible for those crimes that they have committed," he added. "Those crimes have to be investigated."

When asked about the crimes committed by the Americans, Peskov refused to comment, asserting that they would not be protected by the Geneva rules on prisoners of war.

Read more: Ukrainian forces record videos of torturing Russian POWs to incite fear: Russia

"They are not (in the) Ukrainian army, so they are not subject to the Geneva Conventions," he explained.

Drueke and Huynh's social media recordings were shown by a Russian state TV program last week.

The circumstances behind the two men's detention were unclear, as was who was detaining them. Peskov would only state that "authorities" were holding them.

"It depends on the investigation," Peskov said of whether they may face the death sentence.

US President Joe Biden said on Friday that he had no idea where Drueke and Huynh were. They are thought to have joined other foreigners in serving alongside Ukrainian forces.

On Saturday, a US State Department official acknowledged that American authorities had viewed images and recordings of the two US nationals who had been "reportedly captured by Russia's military forces in Ukraine."

Read more: Third US fighter goes missing in Ukraine

"We are closely monitoring the situation and our hearts go out to their families during this difficult time," a spokesperson told AFP.

Regarding Brittney Griner, who is being held in Russia over drug charges, Peskov denied she was being held 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 Spokesperson said.

She was detained at Moscow airport in February on charges of carrying vape cartridges with cannabis oil in her luggage, which has a possible 10-year sentence in prison.