Myanmar freed dozens of detained activists on Tuesday, officials said, after vowing earlier this year to release all prisoners of conscience by the end of December.

Some 56 inmates were set free, the latest in a series of releases that have been seen internationally as a key marker of the country's emergence from military rule.

President Thein Sein, who left Tuesday for a regional summit in Brunei, announced during his first visit to London in July that there would be "no prisoners of conscience in Myanmar" by the end of the year.

Activists said many of those freed Tuesday were linked to ethnic minority rebel groups, with whom the government is holding peace negotiations.

"Our government will release 56 political prisoners," said presidential adviser Hla Maung Shwe, who is also a key figure in the peace talks.

Thein Sein, a former general who took power in March 2011, has earned plaudits and the removal of most western sanctions for reforms that included freeing hundreds of critics detained under the previous junta.

But activists say authorities are continuing to prosecute dissidents and scores remain behind bars. They accuse the government of using the headline-grabbing releases for political gain and leverage with the international community.

Arbitrary imprisonment was a hallmark of the previous brutal junta, which denied the existence of political prisoners even as it meted out harsh punishments to rights activists, journalists, lawyers and performers.

But the nation has since undergone dramatic change, including the release of opposition leader Aung Sang Suu Kyi from long years of house arrest and her election to parliament.

Numbers for political prisoners held in Myanmar vary, but Suu Kyi's opposition said there were around 140 activists held before Tuesday's announcement.

Activist Thet Oo said his organisation, Former Political Prisoners, estimates that around 50 new activists have been held by the current regime.

"Twenty of them are in the prisons and the rest are facing trials. Most of them were charged for protesting without permission and under charges of defaming the state," he said.

Myanmar analyst Richard Horsey said the government was still arresting and detaining activists, but this was "generally in a transparent way unlike the past".

He said recent detentions have often been in "accordance with a law -- even if it's a law that has provisions that aren't consistent with democratic freedoms".

"So the key will be, at the end of the year, has Thein Sein met his pledge on having no political prisoners, and how are the more recently arrested people classified?" he said.

Many of those released Tuesday are believed to be from the country's armed ethnic minority groups in the northern state of Kachin and the eastern state of Shan, according to Thet Oo.

The inclusion of jailed members of the Kachin Independence Army comes amid crucial three-day peace negotiations with the rebels.

Talks, which are being observed by the United Nations and are set to continue to Thursday, are part of government efforts to secure a historic nationwide ceasefire.

As part of the reforms Thein Sein's government has reached tentative peace deals with major armed ethnic minority rebel groups in the country, which has been racked by civil wars since independence from British colonial rule in 1948.

But fighting in Kachin near the northern border with China has continued since a 17-year ceasefire broke down in June last year, leaving tens of thousands displaced.

Myanmar released some 70 political prisoners in July, many of whom were also from Kachin groups.