The US welcomed the release of a naturalised US citizen and democracy activist who had been jailed since September in the Asian country.

Nyi Nyi Aung was freed by a Myanmar court Thursday and put on a flight to Bangkok. He had been sentenced to a three-year jail term in February after being convicted on charges of forgery, illegal possession of foreign currency and refusing to revoke his Myanmar passport.

'We welcome this small but positive step,' said US State Department spokesman Gordon Duguid in Washington.

US officials had been 'working intensely' with Myanmar officials to get Nyi Nyi Aung released.

The US does not have diplomatic relations with Myanmar, which has been under military rule since 1962. An estimated 2,100 political prisoners remain in Myanmar jails or under house arrest.

Nyi Nyi Aung, once a student activist in Myanmar, fled to Thailand after the 1988 crackdown on the fledgling pro-democracy movement. He later immigrated to the US, where he became a naturalised citizen in 2005 and continued campaigning for democracy in Myanmar.

Nyi Nyi Aung was arrested Sep 3 while trying to enter Myanmar at Yangon International Airport. He reportedly entered Myanmar four other times on his US passport between 2005 and 2009, meeting with various dissident groups.