US Secretary of State Hillary Clinton warned Saturday that tensions in the South China Sea threatened peace, while expressing caution over apparent progress in North Korean nuclear talks.

Addressing a regional forum in Indonesia amid efforts by Washington to demonstrate to its Asian allies that the United States would not be eclipsed by rising China, Clinton spoke out on a range of flashpoint regional issues.

On the South China Sea, where China's neighbours have complained of increasing aggression by Chinese naval forces, Clinton said "recent incidents" threatened "peace and stability" in the region.

"These incidents endanger the safety of life at sea, escalate tensions, undermine freedom of navigation, and pose risks to lawful unimpeded commerce and economic development," she told foreign ministers from the Asia Pacific.

In her prepared remarks to the ASEAN Regional Forum, Clinton also said the United States was "encouraged" by rare and unexpected meetings between senior officials from North and South Korea this week in Bali.

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

They also raised the prospect that a compromise could be reached allowing for a re-start of the six-nation talks that are aimed at convincing the North to abandon its nuclear programme.

But Clinton put the onus on the North for any resumption of the talks, saying it had to end "provocative actions".

The United States also issued a statement with its allies South Korea and Japan after their foreign ministers met in Bali saying the North must address its uranium enrichment programme before the talks could restart.

The six-nation talks involve the two Koreas, China, the United States, Japan and Russia. The last round ended in a familiar stalemate in December 2008.

On Myanmar, Clinton urged the government to release some 2,000 political prisoners and open a "meaningful" dialogue with the political opposition after elections last year seen as a sham designed to perpetuate military power.

The Philippines, one of the United States' most enduring Asian allies, repeatedly appealed to Washington ahead of the forum to stand firmly with it amid its spat with the Chinese over the South China Sea.

China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping claims to all or parts of the sea, including hundreds of islets and reefs mostly located in the Paracel and Spratly archipelagos.

The Philippines and Vietnam have recently expressed anger over what they call China's increasingly aggressive actions in the sea, such as harassing their fishermen and oil exploration vessels.

Clinton emphasised to the forum delegates that the United States had a "national interest" in the South China Sea, repeating a line that angered the Chinese at the last ASEAN Regional Forum a year ago in Hanoi.

Chinese Foreign Minister Yang Jiechi responded to the renewed pressure in Bali by insisting China would work to ensure tensions over the dispute did not escalate into conflict, and "guaranteed" freedom of navigation.

"South China Sea and Asia as a whole are peaceful and stable, and this will continue through our joint efforts," Yang told reporters on the sidelines of the forum.

"China will continue to contribute to peace and stability in Asia."

But Manila was not mollified, repeating its position that China's claims to all of the sea were baseless and merely a grab for oil and gas resources.

"I think there is that concern that China is becoming more powerful," Philippine Foreign Secretary Albert del Rosario told AFP.

"It is our expectation that their strength and their growth and their influence will be exercised in a responsible way."

China and the 10 members of the Association of Southeast Asian Nations agreed earlier in the week on a set of guidelines for an eventual code of conduct in the sea.

But the Philippines complained that the eight-point document lacked teeth and said it would continue to seek UN arbitration.

Shi Yinhong of Beijing's Renmin University told AFP the meetings in Bali would help to ease tension in the South China Sea. But he added: "The differences in the positions of the various sides remain."