The death of Thet Win Aung illustrates the high price that opponents of Myanmar's ruling junta have to pay for demanding their human rights, State Department spokesman Tom Casey said.
The 34-year-old opposition activist died this week in jail in Yangon, the capital, where he was sentenced in 1998 to 59 years in jail for agitating for reform of the Asian country's educational system.
Casey said the United States is deeply troubled by Thet Win Aung's death, which he said "demonstrates the tragic price the people of Burma are forced to pay for opposing the repressive policies of the regime and standing up for their human and democratic rights."
He denounced the continued detention of six democracy advocates arrested last month -- Min Ko Naing, Ko Ko Gyi, Htay Kywe, Min Zeya, Pyone Cho and Myint Aye -- and demanded their immediate and unconditional release along with political party leader Aung San Suu Kyi and all other political prisoners.
Human rights groups estimate that Myanmar's prisons hold more than 1,100 political prisoners in brutal conditions.
A group of former political prisoners based near the Thailand-Myanmar border says more than 125 political prisoners have died in jail since 1988, when the current junta took power.
Myanmar, also called Burma, has been under military rule since 1962.