A Russian court on Thursday sentenced the US-Russian dual national Ksenia Khavana to 12 years in prison on a treason conviction for allegedly raising money for the Ukrainian military.

The rights group the First Department said the charges stemmed from a $51 (£40) donation to a US charity that helps Ukraine.

Khavana, whom Russian authorities identify by her birth name of Karelina, was arrested in Ekaterinburg in February. She pleaded guilty in her closed trial last week, news reports said.

Her lawyer, Mikhail Mushailov, said he planned to appeal, adding that Khavana did not plead guilty to all charges.

“She admitted guilt in part in transferring the funds, but did not admit her intent to transfer the funds to the organisations where they were most likely received,” Russia’s Interfax news agency quoted him as saying.

Khavana’s boyfriend, Chris van Heerden, denounced the verdict in an interview with CBS on Thursday and described how he had bought her airline ticket to Russia despite being “uncomfortable” about her return home.

“This whole trial is bogus,” he told the network’s CBS Mornings programme. “After being sentenced to 12 years, I’m trying to make sense of this.

“She’s an American citizen that made a $51.80 cent donation as an American citizen in Los Angeles, [to] exercise her first amendment rights. She has done nothing wrong. The donation was to Ukraine, so she’s been charged with treason for funding the enemy state.”

He said Khavana had made the donation at a humanitarian event in 2022 in Los Angeles but was charged under a Russian law passed the following year.

“Why are we in this position? Ksenia should be home, and I’m angry. I’m angry, and I’m trying to hold my composure … She’s an American citizen making a donation on American soil, being charged now for 12 years in prison. What is not wrongful about that?”

Khavana reportedly obtained US citizenship after marrying an American and moving to Los Angeles. She had returned to Russia to visit her family.

Van Heerden told CBS that he had tried to persuade her against the trip last December but relented after she had insisted she wanted to go.

He accompanied her as far as Istanbul where, he said, she had been scrutinised and closely questioned by a Russian airline official at the airport before departure.

Russia’s federal security service said she “proactively collected money in the interests of one of the Ukrainian organisations, which was subsequently used to purchase tactical medical supplies, equipment, weapons and ammunition for the Ukrainian armed forces”.

Mushailov, her lawyer, disputed that charge, saying his client had not consciously sought to help Ukrainian forces.

“She did not assume that the funds she transferred would be used for anti-Russian actions,” he said.

Since sending troops into Ukraine in February 2022, Russia has cracked down on dissent and passed laws that criminalise criticism of the war and remarks considered to discredit the Russian military. Concern has risen since then that Russia could be targeting US nationals for arrest.

In the largest Russia-west prisoner exchange since the end of the cold war, Russia this month released the Wall Street Journal reporter Evan Gershkovich and American corporate security executive Paul Whelan, who were imprisoned on espionage convictions, and the US-Russian dual national Alsu Kurmasheva, a Radio Liberty/Radio Free Europe journalist sentenced to six-and-a-half years for spreading “false information” about the Russian military.

Van Heerden said he had raised Khavana’s case with the state department but had been told there would be no prisoner swap before the US presidential election in November.

“I was pushing for the last eight months, I was pushing for a wrongful detention so that when we do have a prisoner swap that Ksenia would be on that list and maybe made a priority,” he said.

“I was slowed down. I was told we have time. I was told: ‘Chris, don’t worry, there will be no prison swap until after the election or maybe early next year, so we have time.’ Guess what? There was a prison swap two weeks ago and Ksenia was not on that list. How did we slip up?”

Russia also released several prominent opposition figures who were imprisoned for criticising the war in Ukraine.