Trevor Reed, who was released by Moscow in an April 2022 prisoner swap, has been sent to Germany for the treatment of unspecified injuries incurred while fighting for Ukraine, said Vedant Patel, a State Department spokesman.

Patel stressed that Reed "was not engaged in any activities on behalf of the US government," but had traveled to Ukraine to join the fight on his own.

He said Reed was transported to Germany with the help of a private, non-governmental organization and is currently receiving medical care.

US Secretary of State Antony Blinken said he had no further information on Reed's condition or status but he warned American citizens against heading to Ukraine.

"It underscores why we continue to call on Americans not to travel to Ukraine and not to fight there," he told a news conference during a visit to Tonga.

Blinken said he did not expect Reed's case to have an impact on US efforts to bring home two Americans held in Russia -- Wall Street Journal reporter Evan Gershkovich and former US Marine Paul Whelan.

"My expectation is that even as we are dealing with all sorts of other challenges in our relationship with Russia, we will and we are determined to continue to work to bring both Evan and Paul home," he said.

Reed was a Texas university student in 2019 when he traveled to Russia with his Russian girlfriend.

He was arrested for assault on law enforcement officers after getting drunk, and was sentenced to nine years in prison.

In April last year Russia released him after the White House negotiated an exchange for Konstantin Yaroshenko, a Russian pilot and alleged drug smuggler who was sentenced to 20 years in prison by an American court.

White House Press Secretary Karine Jean-Pierre said the US government has been "extraordinarily, extraordinarily explicit" in warning citizens "not to travel to Ukraine, let alone participate in fighting."

She said she could not provide an estimate of how many Americans might be volunteering on the side of Ukraine against Russian forces.

Asked if the Reed episode could complicate US efforts to procure the release of other Americans imprisoned -- unjustly, according to Washington -- in Russia, Jean-Pierre said the cases were "separate. They're not the same."