By Aung Hla Tun

YANGON (Reuters) - A new official human rights body in Myanmar urged the president on Tuesday to release "prisoners of conscience" in an open letter in state media, the clearest sign yet that the reclusive state may free political prisoners within days.

The United States, Europe and Australia have made the release of an estimated 2,100 political prisoners a key condition before they would consider lifting sanctions imposed in response to human rights abuses, including a bloody army crackdown on a 1988 student uprising.

One lawmaker in Myanmar's parliament, who attended a meeting on Friday in the capital, Naypyitaw, told Reuters the release of political prisoners could come "in a few days." He said that was the message given by Shwe Mann, the Lower House speaker.

Prisoners who did not pose "a threat to the stability of state and public tranquility" should be released, Win Mra, chairman of Myanmar National Human Rights Commission, wrote in the open letter published on Tuesday.

"The Myanmar National Human Rights Commission humbly requests the president, as a reflection of his magnanimity, to grant amnesty to those prisoners and release them from the prison," the letter ended.

The commission was formed last month by the president.

The statement is one of several recent signs of change since the army nominally handed over power in March to civilians after elections in November, a process ridiculed at the time as a sham to cement authoritarian rule behind a democratic facade.

Recent overtures by the government hint at deeper changes at work -- from calls for peace with ethnic minority guerrilla groups to some tolerance of criticism and more communication with Nobel Peace Prize laureate Aung San Suu Kyi, who was released last year from 15 years of house arrest.

Last week, the government suspended a controversial $3.6 billion, Chinese-led dam project, a victory for supporters of Suu Kyi and sign that the country was willing to yield to popular resentment over China's growing influence.

These moves have stoked hopes the new parliament will slowly prise open the country of 50 million people that just over 50 years ago was one of Southeast Asia's wealthiest as the world's biggest rice exporter and a major energy producer.

The open letter marks a significant shift in the former British colony, also known as Burma, where authorities have long refused to recognize the existence of political prisoners, usually dismissing such detainees as common criminals.

PHASED PRISONER RELEASE?

Nestled strategically between economic powerhouses India and China, Myanmar has been one of the world's most difficult for foreign investors, restricted by sanctions, blighted by 49 years of oppressive military rule and starved of capital despite rich natural resources, from gemstones to timber to oil.

In November 2009, U.S. President Barack Obama offered Myanmar the prospect of better ties with Washington if it pursued democratic reform and freed political prisoners, including opposition leader Suu Kyi.

But Washington's demands go beyond prisoners, making it unclear whether it would lift sanctions if the prisoners are released and Suu Kyi withdraws her support for sanctions.

The United States has also demanded more transparency in Myanmar's relationship with North Korea and end to human-rights abuses involving ethnic minorities in its remote border regions with Thailand and China.

A European diplomat in Bangkok, however, said many European countries had privately urged the European Union to ease sanctions and that the EU could face strong internal pressure to do so if prisoners are released and Suu Kyi changes her stance.

"All it would take is for Suu Kyi to urge sanctions to come down," said the diplomat, who declined to be identified because of the sensitivity of the subject.

Some speculate the release could come on Wednesday, the Thadingyut Full Moon, an auspicious day in the Buddhist-majority country. But President Thein Sein is leaving for India on that day, so the announcement may wait until his return.

Myanmar also appears to be trying to convince the 10-member Association of South East Asian Nations (ASEAN) to allow it to take its rotating presidency in 2014, two years ahead of schedule and a year before the next general election.

Hosting ASEAN could give Myanmar a degree of international recognition and help convince the World Bank and other multilateral institutions to return to the impoverished country.

Myanmar has invited a delegation from the International Monetary Fund to visit this month to advise on currency reform.

It is unclear whether Myanmar's authorities will release all political prisoners at once.

"The government is in a dilemma," said a Southeast Asian diplomat in Yangkon. "They know the release of political prisoners will definitely help improve their image, which they desperately need to secure the alternate seat of the ASEAN chair. However, they are very much worried that some released prisoners can make their situation difficult.

"I think they will take a calculated risk and release some prominent ones but only the less active ones in the first batch and then the rest some time later."

He expected the release to start before Indonesian Foreign Minister Marty Natalegawa arrives in Myanmar next week.

According to the Thai-based Assistance Association for Political Prisoners (Burma), there were over 2,100 prisoners of conscience as of last month.

"That data is quite reasonable since they have secret access to reliable sources," said Aung Thein, a legal expert and prominent Burmese human rights activist.

"There are a number of forgotten political prisoners, especially of ethnic minorities, who have been behind bars since 1988. Some have not seen any visitors at all."

Among the most prominent are Shan State ethnic leader Khun Tun Oo plus political activists Min Ko Naing and Ko Ko Gyi, both members of the so-called "88 Generation Students Group," still-influential leaders of a mass uprising against Myanmar's former military rulers in 1988.

(Writing and additional reporting by Jason Szep; Editing by Alan Raybould)