Myanmar has pardoned hundreds of prisoners under an amnesty that appears to be a goodwill gesture just days before a visit by US President Barack Obama.
The government ordered the release of 452 prison inmates on Thursday in a move criticised by pro-democracy activists for allegedly failing to grant freedom to many political detainees.
It was not immediately clear if any jailed dissidents were among those given amnesty, prompting rights groups to renew calls for officials to bring transparency to one of the world's most opaque prison systems.
Myanmar has long insisted that all prisoners are criminals and release no official information on political detainees.
"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.
State media said some of the prisoners to be released are foreigners who will be extradited, but gave no details.
The move came in advance of a visit on Monday by Obama, who will become the first sitting US president to visit the once-pariah nation.
Previous releases in Myanmar, which appear to be a part of wave of political and economic reforms, have helped convince Western nations, including the US, to ease sanctions on the country.
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 favour after almost five decades of repressive military rule.
Under the former 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, the leader of the National League for Democracy party.
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 said.
Ohn Kyaing, another spokesman for the NLD, questioned the timing of the latest mass pardon.
"It is strange that they released prisoners just before Obama's visit," Ohn Kyaing told the AFP news agency.
"They should have done it before and showed their genuine will to give the amnesty," he said, adding that it was unclear if any NLD members were among those being freed.
Human Rights Watch (HRW), the New York-based watchdog group, 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?'' Phil Robertson, the group's deputy Asia director, said.
"This whole process is being drawn out unnecessarily to maximise 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 towards democracy, easing harsh media censorship, signing ceasefire 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 commonly implicated in rights abuses.