* Opposition party believes many political prisoners included
* Amnesty could boost ties between Myanmar and United States
* Myanmar eager for import bans to be lifted
By Aung Hla Tun
YANGON, Sept 17 (Reuters) - Myanmar pardoned more than 500 prisoners on Monday in an amnesty that included political detainees, according to the opposition party, a step that could strengthen the former military state's growing bonds with Washington.
A government bulletin announcing the news on state television did not make clear if any of those affected were political inmates. But Naing Naing, an official of Aung San Suu Kyi's National League for Democracy (NLD) party, said he was hopeful the amnesty included the country's 424 remaining political prisoners.
"We're optimistic that these are the remaining political prisoners," said Naing Naing, himself a former political prisoner.
He said the NLD received word that political prisoners had been amnestied from the Assistance Association for Political Prisoners (AAPP), a Thai-based group that tracks prisoners in Myanmar, also known as Burma.
Contacted by Reuters, Bo Kyi, secretary-general of the AAPP, said political prisoners were among those who had been pardoned, but the organisation needed more time to confirm the number.
The timing of the amnesty is significant coming ahead of a U.S. visit by Thein Sein, Myanmar's reformist president. The State Department said U. S. officials will meet Thein Sein on the sidelines of the U.N. General Assembly next week.
It also coincides with a separate U.S. trip that began on Monday b y Suu Kyi, the 1991 Nobel Peace Prize winner.
Suu Kyi's election to parliament in April helped to transform Myanmar's pariah image and convince the West to begin rolling back sanctions after a year of dramatic reforms, including the release of about 700 political prisoners in amnesties between May 2011 and July.
The United States has repeatedly called for all remaining dissidents to be freed as a pre-condition for further economic rewards, including a relaxation of a ban on imports of Myanmar-made products imposed years ago in response to human rights abuses.
In Washington, the U.S. State Department reacted cautiously to news of the amnesty and said it would monitor events to see whether any political prisoners were in fact released.
"We have seen reports that the government of Burma's Information Ministry has announced that 514 prisoners will be granted Presidential Amnesty on humanitarian grounds," said a State Department spokeswoman on condition of anonymity.
"We are watching developments of the prisoner release closely and will work carefully to verify if any political prisoners are released," she added. "The United States continues to call for the immediate and unconditional release of all political prisoners."
Naing Naing of the NLD said the 424 freed political prisoners excluded inmates who were former military intelligence officials. The military ruled the country for 49 years as one of Asia's most oppressive regimes before ceding power to a semi-civilian government in March last year.
Suu Kyi left Sunday for the United States, where she will receive a congressional medal.
Thein Sein, a former general, was due to head to the United States on Sept. 24, where he will address the United Nations General Assembly in New York for the first time as president.