WASHINGTON- The United States on Wednesday demanded that Myanmar release an American rights activist handed a three-year prison term, with one lawmaker calling for consideration of tougher sanctions.

A court in the military-ruled nation sentenced Kyaw Zaw Lwin, who also goes by Nyi Nyi Aung, on fraud and forgery charges.

It came as the United States engages in a cautious dialogue on improving relations.

"The United States is deeply concerned by the Burmese authorities' decision today to convict US citizen Kyaw Zaw Lwin on politically motivated charges," State Department spokesman Philip Crowley said in a statement.

"We consider these actions unjustified. We continue to urge the Burmese regime to release him and allow him to return home to the United States," Crowley said, using the former name of the Southeast Asian nation.

The court sentenced the Myanmar-born US citizen on charges of fraud and forgery, in part for not renouncing his earlier citizenship.

His supporters say that the charges were trumped up by the regime as it seeks to consolidate rule ahead of elections later this year, which the opposition fears is an attempt to legitimize military rule.

Representative Howard Berman, who heads the House Foreign Affairs Committee, called for President Barack Obama's administration to consider tightening sanctions on Myanmar.

"The sentencing of Nyi Nyi Aung is a serious impediment to improved relations with the United States at a time when our country has embarked on a new approach toward Burma," said Berman, a member of Obama's Democratic Party.

The Obama administration launched a dialogue with Myanmar last year in hopes of coaxing it out of isolation.

It left wide-ranging sanctions in place but said they could eventually be lifted in return for democratic progress.

"Months after this new approach was announced, it is disappointing that the junta has failed to respond to formal diplomatic complaints regarding his reported severe mistreatment, including allegations of torture and repeated,
lengthy denials of access to consular services through the US embassy," Berman said in a statement.

He was one of more than 50 US lawmakers who wrote to the junta warning of repercussions if it sentenced Kyaw Zaw Lwin.

Nyi Nyi Aung's fiancee Wa Wa Kyaw, who works as a nurse just outside Washington in the eastern state of Maryland, was hopeful that the United States would push his case.

"I hope he can come home soon and we can be reunited. I can't wait," she told AFP.

"If the Burmese regime is really moving towards democratic change, this illegal imprisonment and oppression should stop. Keeping an American prisoner can only impede the US-Burma engagement policy."

She said she was not surprised by the conviction in light of Kyaw Zaw Lwin's political activities, but credited US and foreign pressure with preventing an even harsher sentence.

"It's a shame that he was here last year campaigning for the release of all Burmese political prisoners and now he has become a political prisoner himself," she said.

The junta's most famous prisoner is democracy icon Aung San Suu Kyi, whose party won 1990 elections but was never allowed to take office.

The Nobel laureate lives under house arrest.