NUSA DUA, Indonesia, July 23, 2011 (AFP) - The United States said Saturday it was "encouraged" by unexpected and rare talks between North and South Korea over Pyongyang's nuclear programme.

But US Secretary of State Hillary Clinton told regional foreign ministers at an Asian security forum in Indonesia that the onus remained squarely on the North to prove its sincerity before six-party talks could resume.

"We are encouraged by the recent North-South meeting that took place on the margins of the ASEAN Regional Forum, but we remain firm that in order for six-party talks to resume, North Korea must take steps to improve North-South relations," she said in prepared remarks.

Clinton urged North Korea to "demonstrate a change in behaviour, including ceasing provocative actions" and take steps towards denuclearisation and meeting its obligations under previous agreements and UN resolutions.

Her comments came after South Korean nuclear envoy Wi Sung-Lac and his counterpart from the North, Ri Yong-Ho, met for more than two hours at a luxury hotel in Bali on Friday.

Both emerged saying they hoped to re-start stalled six-party talks.

South's foreign minister, Kim Sung-Hwan, then briefly met his North Korean counterpart, Pak Ui-Chun, on the sidelines of the forum on Saturday morning, according to South Korea's Yonhap news agency.

The contacts offered hope that the rivals may be starting to mend ties after more than a year of high tensions, including the North's shelling of a South Korean island and alleged torpedoing of a South Korean warship.

The six-nation talks, involving the two Koreas, China, the United States, Japan and Russia, are a tortuous process aimed at convincing the North to give up its nuclear programme in return for diplomatic and economic rewards.

The last round ended in a familiar stalemate in December 2008. The North formally abandoned them in April 2009, blaming alleged US hostility for its withdrawal, and staged its second nuclear test a month later.

North Korea had previously refused to discuss its nuclear programme with the South alone, but finally relented after lobbying from various six-nation parties.

Six-party host China had been pushing for an inter-Korean nuclear meeting, followed by US-North Korean talks, to pave the way for a resumption of the full dialogue.

US officials travelling with Secretary of State Hillary Clinton in Bali also said Washington had been "deeply engaged" with the South over the past few weeks to bring the two sides together.

"The next step is for the United States to have extensive discussions, first with the South Koreans and then with the Japanese. And then at that time we will make a determination on the way forward," a US official said.

"There's no determination to rush into anything... We need to see a clear and sincere indication on the part of the North that they are prepared to work constructively with the South."