MOSCOW (AFP) - A Russian state TV channel aired videos on social media of two Americans who went missing last week while fighting alongside the Ukrainian army, stating they had been captured by Russian forces.

United States President Joe Biden had said earlier on Friday (June 17) he did not know the whereabouts of Mr Alexander Drueke and Mr Andy Huynh, both US military veterans whose relatives lost contact with the pair.

The missing Americans - including a third identified as a former US Marines captain - are believed to be part of an unknown number of mostly military veterans who have joined other foreigners to volunteer alongside Ukrainian troops.

On Friday evening, Russian journalist Roman Kosarev - who works with state TV RT channel - posted a video on messaging platform Telegram of Mr Drueke speaking facing the camera.

"Mum, I just want to let you know that I'm alive and I hope to be back home as soon as I can be," said Mr Drueke, who was seated in what appeared to be an office and dressed in military fatigues.

"Love Diesel for me, love you," he said, concluding his brief video with a quick wink. Reports in the US say Diesel was Mr Drueke's dog.

RT's official Telegram channel also posted an interview with Mr Huynh, in which he said the duo had been "engaged in combat with Russian troops" near Ukraine's flashpoint Kharkiv area.

After the pair retreated and hid for hours, they surrendered themselves to Russian troops, Mr Huynh said.

The pair were also filmed in separate RT videos - directly facing a camera angled from above - saying "I'm against the war", in poor Russian.

The circumstances under which the two men were speaking were not fully clear, nor who specifically was holding them.

A US State Department spokesman 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 closing monitoring the situation and our hearts go out to their families during this difficult time," the spokesman told AFP.

During a White House briefing on Friday, Mr Biden urged US citizens not to go to Ukraine.

"Americans should not be going to Ukraine now. I'll say it again: Americans should not be going to Ukraine," he said.

The Russian proxy authorities in the so-called Donetsk People's Republic, a Moscow-controlled swathe of eastern Ukraine, have sentenced to death two British men and a Moroccan captured earlier in fighting.