Burma ordered 452 prisoners freed Thursday in an apparent goodwill gesture ahead of a historic visit by President Barack Obama, but activists criticized the move for failing to focus on hundreds of political detainees.

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 prisoners walk outside Insein prison in Yangon, Myanmar after they were released Thursday. (Khan Maung Win/Associated Press)

Burma'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 Myanmar. 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 centrepiece of its reforms over the last year and a half to seek international favour 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.

More than 300 political prisoners still held

Under the now-defunct junta, rights groups said more than 2,000 activists and government critics were wrongfully imprisoned.

Myanmar's main opposition movement estimates that at least 330 political prisoners remain incarcerated, according to Nyan Win, a spokesman for Aung San Suu Kyi's National League for Democracy party.

'This whole process is being drawn out unnecessarily to maximize the Burmese government's leverage with the international community'—Phil Robertson, Human Rights Watch

Nyan Win said he believes the latest release is "a goodwill gesture" ahead of Obama's trip. "We want all political prisoners to be freed," he added.

New York-based Human Rights Watch accused the government of using strategically timed prisoner releases to appease the international community.

"The government of Burma has said they are committed to releasing all political prisoners. So why haven't they?" said Phil Robertson, the group's deputy Asia director. "This whole process is being drawn out unnecessarily to maximize the Burmese government's leverage with the international community."

The last release took place in September, a week before Thein Sein visited New York for the UN General Assembly.

Thein Sein's government has spearheaded a major transition toward democracy, easing harsh media censorship, signing cease-fire deals with armed rebel groups, and opening the country more to Western investment.

But rights groups say Thein Sein has not yet consolidated the political and economic reforms. The military is still dominant and is implicated in rights abuses. It has failed to prevent outbreaks of communal violence in the west of the country that have left scores dead.