The family of one of the former US military members captured in Ukraine say that his captors are looking to negotiate his release.

Alex Drueke’s family also said that he has twice told the US State Department that he was taken by members of the Donetsk People's Republic - not the Russian army. The Donetsk People's Republic is a Russian-backed region in Ukraine.

Mr Drueke, 40, and Andy Huynh, 27, went missing in Ukraine while volunteering with other foreign fighters who joined the in the effort to repeal Russia's invasion of the country. The men were last seen disabling a Russian tank. After the smoke from the battle cleared, their unit members could not find them and Russian forces later claimed on social media that they had captured a pair of Americans.

Mr Drueke's aunt, Dianna Shaw, told ABC News that he called the State Department on Saturday from a Russian phone number. He reportedly read a statement saying the DRP wanted to negotiate the release of himself and Mr Huynh. He also added that he was being treated well and was being given shelter and water.

On Tuesday, Mr Drueke called again, reading a second statement regarding negotiations but without mentioning the terms of any possible negotiation. He said that he was spending most of his time in solitary confinement and that he had not seen Mr Huynh in several days.

"We have been in contact with the Ukrainian and Russian authorities regarding U.S. citizens who may have been captured by Russia's proxies while fighting in Ukraine," the State Department told ABC News. "We are seeking to learn as much as we can and are providing every form of support possible, including for the families, with whom we are in contact. Due to privacy considerations, we have no further comment at this time."

The State Department also said it advises Americans to stay away from the fight, as they could be captured or killed.

Ukrainian President Volodomyr Zelenskyy called the men "heroes" and said he was committed to securing their release.

"We'll fight for them and get them back, and of course they will come back to their families," he said in a statement. "To me, it is a great honor that in the world there are some soldiers that are not afraid, and they came to support us and our sovereignty and independence."

Mr Drueke's mother, Bunny Drueke, issued a statement saying it was "very encouraging to hear the Ukrainian government is committed to securing Alex and Andy's release."