Phuket, Thailand - US Secretary of State Hillary Rodham Clinton on Wednesday signed South-East Asia's main friendship and security treaty in a gesture signalling America's return to the region.

"The United States is back in South-East Asia," Clinton told a press conference minutes before signing the Treaty on Amity and Cooperation in South-East Asia (TAC), a loose code of conduct for the region that was signed by the original founding members of the Association of South-East Asian Nations (ASEAN) in 1976.

Nearly all of ASEAN's main dialogue partners, such as Australia, China, India, Japan and South Korea, have already signed the TAC.

The US, under the previous administration of George W Bush, refused to accede to the document to fear its would cramp its unilateral-inclined style of diplomacy in Asia.

"US President (Barack) Obama and I believe this region is vital to global progress, peace and prosperity," Clinton said before signing the TAC.

"The treaty seals our commitment to work in participation with ASEAN," she added.

Clinton was in Phuket to sign the TAC and to attend the ASEAN Regional Forum (ARF), Asia's main security event that has drawn 27 foreign ministers to the island, on Thursday.

The forum is expected to focus on the region's two main security threats - North Korea's nuclear armaments programme and Myanmar's ongoing political instability.

Clinton on Wednesday called for "irreversible denuclearization" in North Korea as the only "viable path" for North Korea.

She also called on Myanmar to free its political prisoners including Aung San Suu Kyi and take other steps to assure a credible general election next year. (dpa)