Governments, including the US, former colonial ruler Britain and Germany have condemned the extension.

The news broke during a summit of Southeast Asian nations in Laos as the leaders welcomed Myanmar's release of more than 9,000 inmates from jail.

Annan said her detention "is not in the interest of the process of national reconciliation and democratization in Myanmar" and called for her to be freed as soon as possible, the UN chief's spokesman Fred Eckhard said in a statement on Tuesday.

The UN chief said the junta should "seize the momentum" created by the release of the 9,248 detainees by freeing Aung San Suu Kyi and other political detainees as soon as possible.

The United States warned Tuesday it might boycott meetings of the Association of Southeast Asian Nations (ASEAN) when Myanmar takes over the chairmanship of the 10-nation grouping in 2006, unless the regime improves its human rights record.

"The US has made clear that the situation in Burma has complicated our dealings with ASEAN," State Department spokesman Richard Boucher said, using Myanmar's previous name.

The United States has repeatedly criticised the junta's human rights record and imposed trade and investment sanctions against the impoverished nation.

At the Vientiane summit, Thai Prime Minister Thaksin Shinawatra and Indonesia's foreign ministry expressed concern at the latest developments.

Britain, which governed Myanmar until its independence in 1948, said it was "deeply disturbed" at the news of the detention extension.

"Her continued detention is completely unjustified and unacceptable, both to the people of Burma and to the international community," said junior Foreign Office minister Douglas Alexander.

"Aung San Suu Kyi and the democratic movement have a key role to play in national reconciliation in Burma."

Germany called for her release along with all other political prisoners.

"The involvement of all political and social players in Myanmar is an indispensable condition for a successful process of democratisation," said junior foreign minister Kerstin Mueller in a statement.