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Associated Press
Mya Aye, center, one of the 88 Generation students activists, gets welcomed by his colleagues as he arrives at Yangon airport after being released from prison Friday, Jan.13, 2012, in Yangon, Myanmar.

WASHINGTON — The United States is restoring full diplomatic relations with Myanmar, a landmark in the Obama administration's drive to reward democratic reforms by a government the U.S. previously treated as a pariah.

The decision announced Friday to exchange ambassadors with Myanmar for the first time in two decades followed the release of hundreds of political prisoners, but Washington probably will be looking for fair conduct in coming elections and an end to ethnic violence before it lifts sanctions.

The U.S. also wants Myanmar to open up to U.N. nuclear inspectors and sever illicit military ties with North Korea because of concerns that Pyongyang has sold Myanmar defense hardware, including missiles, in defiance of international sanctions.

Myanmar President Thein Sein pardoned 651 detainees on Friday, among them leaders of brutally repressed democratic uprisings, heads of ethnic minority groups, journalists and even a former prime minister who had been blamed himself for incarcerating activists.

President Barack Obama, in a statement, described the pardons as "a substantial step forward for democratic reform."

The U.S. decision follows a historic visit by Hillary Rodham Clinton in December, the first by a secretary of state in 56 years, as a way to deepen engagement and encourage more openness in the country. That is part of a broader administration policy to step up U.S. involvement across the Asia-Pacific region as well as a way to counter the growing influence of China, which has remained Myanmar's core ally during its decades of isolation.

"As I said last December, the United States will meet action with action," Clinton said at the State Department. "Based on the steps taken so far, we will now begin."

The highest-level U.S. diplomat based in Myanmar, also known as Burma, has been a chargé d'affaires rather than an ambassador. Washington downgraded its representation in 1990, when opposition leader Aung San Suu Kyi's party swept elections but was barred from power by the military.

Myanmar's own diplomatic representation in Washington also currently is a step below the level of ambassador.