WASHINGTON (AFP) - The fiancee of an American rights activist imprisoned by Myanmar said she felt betrayed by the US government and urged a more robust response to the military regime.

In a piece to appear in Monday's Wall Street Journal, Wa Wa Kyaw said she and her fiancee Nyi Nyi Aung had been grateful to become US citizens as their adopted country "cherished democracy, freedom and human rights."

"But over the past five months our government has betrayed us," wrote Wa Wa Kyaw, a nurse in the eastern state of Maryland.

Supporters say Nyi Nyi Aung, a democracy campaigner who also goes by Kyaw Zaw Lwin, returned to Myanmar in September to visit his ailing mother, herself a jailed activist.

He was arrested and on February 10 handed a three-year sentence on fraud and forgery charges.

State Department spokesman Philip Crowley has urged the junta in Myanmar, earlier known as Burma, to release him, calling the conviction "unjustified" and "politically motivated."

But Wa Wa Kyaw faulted Secretary of State Hillary Clinton for not doing more on his case before the verdict. President Barack Obama's administration has launched a dialogue with Myanmar aiming to end its isolation.

"President Obama and Secretary Clinton, my message is simple. Neither your words nor your actions show that you take my fiance's imprisonment seriously," she wrote.

"I beg you to stop ignoring his plight, and to help secure his release from this illegal and unjust imprisonment. Just as Nyi Nyi continues to live up to the oath we took to defend America, please, live up to the promise America made to defend us."

The administration opened talks with the junta last year, saying that the previous approach focused on punishing Myanmar has failed. It has maintained sanctions, saying they would be eased in return for progress on democracy.

The military has ruled Myanmar since 1962. It refused to let Aung San Suu Kyi's National League for Democracy take power after it swept the last elections in 1990.

The junta plans to hold new elections this year, which have met with wide skepticism as Aung San Suu Kyi, a Nobel Peace Prize laureate, remains under house arrest.