Myanmar released about 20 political prisoners on Friday, a top official said, hours before its reform-minded leader was due to leave on a landmark visit to the United States to meet President Barack Obama.
President Thein Sein, a former general, has freed hundreds of political detainees since coming to power in early 2011 as part of sweeping changes that have led to the end of most Western sanctions.
Zaw Htay, director of Thein Sein's office, said on Twitter that the latest amnesty showed the president's determination to have an "inclusive political process", denying that the dissidents were being used as "tools".
But activists say some 200 political prisoners remain in jail and accuse Myanmar of using a series of headline-grabbing amnesties for political gain.
"These releases are blatantly designed to get good publicity ahead of Thein Sein's visit to the US. It is disgraceful to use political prisoners for public relations like this," Wai Hnin of Burma Campaign UK said in a statement.
Last month dozens of political prisoners were pardoned a day after the European Union agreed to end almost all sanctions against the former pariah state.
Opposition member Nay Myo Zin, whose previous amnesty was revoked earlier this month to the dismay of campaigners, was among those freed on Friday.
"I was invited to cooperate with the authorities in this reform process," he told AFP by telephone after his release from prison in the Irrawaddy region.
"We have a good foundation for the reform process. We have moved on from the old era," he added.
Thein Sein, who will be welcomed at the White House on Monday, will be the first leader of the former military-ruled nation to visit Washington since 1966.
He was due to leave Myanmar late on Friday and return next Thursday, a Myanmar government official who did not want to be named told AFP.
The former junta premier came to power in the wake of elections marred by widespread allegations of fraud and intimidation.
Under his leadership, Myanmar has eased censorship, taken steps to open the economy and allowed opposition leader Aung San Suu Kyi -- a Nobel peace laureate who spent most of the past two decades under house arrest -- to enter parliament.
The Obama administration has suspended most sanctions against Myanmar as it seeks to encourage the changes and extend its influence in the long-isolated nation.
In an interview with AFP from the capital Naypyidaw on Thursday, Zaw Htay said the president's invitation to the White House was an endorsement of "Myanmar's Spring".
"Myanmar's Spring is more concrete than the Arab Spring. This spring represents the values that the US has been promoting around the world," he said.
Rights groups, however, have decried recent anti-Muslim violence and accused the security forces of failing to stop -- or even supporting -- sectarian attacks.
Nyan Win, a spokesman for Aung San Suu Kyi's National League for Democracy (NLD) opposition party, welcomed the latest prisoner amnesty and said he hoped Thein Sein's visit would help foster better ties with the United States.
"We think his US trip will be a beneficial one for the country because we will have good relations" with Washington, he said.