Burma on Tuesday announced amnesty for 6,359 prisoners, a group expected to include hundreds of political prisoners whose release would be a dramatic sign of an opening after nearly 50 years of autocratic rule.

The United States and Europe have long demanded the release of political prisoners as a condition for lifting economic sanctions on Burma, also known as Myanmar.

In Washington, officials welcomed the announcement as the latest sign that a major shift may be underway in Burma. But they cautioned that much depends on how many of those released prove to be political prisoners — something that may not be known for weeks.

“We’re obviously looking to see who these folks are and to hope that it is a full, and ideally complete, release of political prisoners,” said State Department spokeswoman Victoria Nuland.

The prisoner release and other recent signs of opening are viewed as part of a deliberate Burmese rapprochement with the West, which could serve as a counterweight to the growing and increasingly aggressive power of China, Burma’s longtime close ally.

China was stunned last month when Burmese President Thein Sein halted construction of a mammoth Chinese-financed dam in the north of the country. Thein Sein leaves this week on a trip to India, the world’s largest democracy, which has long competed with China for influence and business deals in Burma.

Another key factor is the possible lifting of Western sanctions that tightly restrict investment, trade and financial transactions in Burma. The Burmese government also seeks increased recognition from other Asian countries, which could lend the current leaders legitimacy by granting Burma the chairmanship of the Association of Southeast Asian Nations.

State-controlled Burmese media said the releases of inmates would begin Wednesday, a Buddhist religious holiday. The news came on the same day that a newly established human rights commission in Burma issued an open letter to Thein Sein, urging him to free “prisoners of conscience” who do not pose a “threat to the stability of the state and public tranquility.”

The announcement did not name those due to be freed, but both supporters and opponents of the regime, as well as diplomats, said they believe the amnesty will cover many pro-democracy activists and others imprisoned because of their political beliefs.

Others were more cautious. Which prisoners are to be released matters more than how many, said Aung Din, executive director of the Washington-based U.S. Campaign for Burma. In the past, the government has announced mass releases of prisoners as often as twice a year, he noted, including one recent release of more than 10,000 people that included only about 50 political prisoners.

Such prisoners, who are scattered in jails across the country, range from former student leaders — some of whom have been in prison since pro-democracy protests in 1988 — to military intelligence officers jailed after the 2004 purge of Khin Nyunt, a former prime minister and spy chief who had tried to reach out to opposition forces.

Their numbers are estimated to range from 600 to more than 2,000. Burma, ruled by military generals in one guise or another since 1962 and long one of the world’s most repressive states, denied for decades that such prisoners even existed.

Thein Sein, an ex-general, took office in March and has since implemented some reforms, passing laws that allow trade unions and opening talks with Nobel Peace Prize recipient Aung San Suu Kyi, Burma’s main opposition leader.

Speaking in Bangkok on Monday, a day before the amnesty news, U.S. Assistant Secretary of State Kurt M. Campbell said there “are clearly changes afoot” in Burma and added that Washington will respond favorably, although he did not spell out how.

Lifting U.S. sanctions — a complicated process — will probably take time, experts said, and could require winning support from Congress. More immediate steps are likely to include an easing of U.S. pressure on the World Bank and International Monetary Fund not to provide financial assistance to Burma.

“The advantage of that is that to get those kinds of loans and aid, it would require the country to open up its books and provide transparency on its economic data,” said Michael Green, a former National Security Council senior official.

An IMF team is due to visit Rangoon, also called Yangon, soon, to advise on how to reform Burma’s chaotic and corrupt foreign exchange system, which includes a plethora of different state-fixed rates.

A single exchange rate would mark an important step in reforming Burma’s decrepit economic system, which has enriched insiders but wrecked what had been, when the country gained independence from Britain in 1948, Southeast Asia’s most prosperous economy.

Tin Oo, vice chairman of the National League for Democracy and a longtime lieutenant of Aung San Suu Kyi, said a release of political detainees would represent an important break with the past, though he noted that previous governments had also issued amnesties. A May amnesty for 1,400 inmates included just a few dozen political prisoners.

The current government, despite being stacked with generals, seems more serious about change than its predecessors, Tin Oo said.

He described Thein Sein as someone Suu Kyi “can work with,” unlike the country’s previous leader, Senior Gen. Than Shwe, who ruled from 1992 until earlier this year.

Since being released from house arrest last year, Suu Kyi has held a series of meetings with the president and senior officials to discuss the release of political prisoners and other issues.

Government censors, who vet all publications, this week allowed her photograph to appear on the front pages of several privately run journals. State-run newspapers have dropped what had been a campaign to demonize her and her supporters in the National League for Democracy.

Taken together, the recent reforms “have gone too far to be just window dressing,” said Steven Marshall, the International Labor Organization’s liaison officer in Rangoon. “The political environment now is very different than what it was before.”

Staff writer William Wan in Washington contributed to this report.