"We urge the government to continue to work expeditiously through the political prisoner review committee to release all political prisoners unconditionally and to remove conditions placed on those already released," State Department spokeswoman Ms Jen Psaki said.
Washington was seeking details about reports that President U Thein Sein had pardoned some 3,073 people including former military intelligence figures, Ms Psaki told reporters.
However, "an estimated 30 to 40 remain incarcerated," she said.
The reformist regime, which is preparing to host a landmark November meeting of international and regional leaders, has granted a series of amnesties as part of dramatic reforms since the end of outright military rule in 2011.
US President Barack Obama is to travel to Myanmar to attend the upcoming East Asia summit, which will happen alongside a meeting of the Association of Southeast Asian Nations (ASEAN) next month.