State media reported today that 23 prisoners were freed, though it did not call them political offenders. Ye Aung, a member of the government's political prisoner scrutiny committee, said at least 20 political detainees were freed yesterday.
President Thein Sein will visit the White House on Monday, the first state visit by a Myanmar leader in almost 47 years.
The US applied sanctions against Myanmar's previous military regime for its poor human rights record. Thein Sein has implemented several reforms since his election in 2010, including freeing hundreds of political prisoners. The US in turn eased most sanctions. In November, Barack Obama became the first US president to visit Myanmar.
Thein Sein pardoned 93 prisoners, including at least 59 political detainees, in April, a day after the European Union lifted sanctions against the Southeast Asian nation.
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The release of political detainees in Myanmar has been a key concern of the United States, and Washington wants all of the country's political prisoners freed.
A group campaigning for democracy in Myanmar which is also known by its old name, Burma accuses Thein Sein's government of using political prisoners for public relations purposes.
The group also expressed concern that Thein Sein's democratic reforms were incomplete.
"Thein Sein has also left almost every repressive law used to jail political prisoners in place," the statement said. "Almost all the releases of political prisoners have only been released conditionally, meaning that if they engage in political activities which the government does not like they can be put back in jail and have to serve a new prison term and their old prison term."
One of those freed yesterday was Nay Myo Zin, a former prisoner who had been amnestied before but who was recently ordered to serve the remainder of his original sentence after he was arrested for participating in a farmers' protest against land confiscation.