Myanmar announces release of 452 prisoners

It was not immediately clear if any political prisoners were among those granted freedom, prompting rights groups to renew calls for the government to shed light on one of the world's most opaque prison systems. Myanmar's government has long insisted that all prisoners are criminals and releases no official information on who is a political prisoner, where they are detained and how many remain jailed.

"This is extremely disappointing because we haven't heard of any political prisoners being released. This is a shame," said U Naing Naing of the Central Social Assistance Committee, which helps families of political prisoners.

Other groups that monitor political prisoners gave similar reports. Many political detainees are in remote areas where communications are difficult, so the extent of the release may not be known for several days.

Past releases have included both prisoners of conscience and common criminals.

The announcement in the state-run New Light of Myanmar newspaper came ahead of a visit Monday by Obama, who will become the first sitting American president to visit the once-pariah nation, also known as Burma. State media said some of the prisoners to be released are foreigners who will be extradited, but gave no details.

The administration of President Thein Sein has made freedom for political prisoners a centerpiece of its reforms over the last year and a half to seek international favor after almost five decades of repressive army rule. Earlier prisoner releases helped convince Western nations, including the United States, to ease sanctions they had imposed against the previous military regime.