US Secretary of State Hillary Clinton issued a warning over tensions in the East Sea, also known as South China Sea, while welcoming progress in Korean talks, as she addressed an Asian security forum.
"The United States is concerned that recent incidents in the [East Sea] threaten the peace and stability on which the remarkable progress of the Asia Pacific region has been built," Clinton told a security forum in Indonesia, on Saturday.
"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."
Tensions have flared in recent months over rival territorial claims in the potentially resource-rich waterway by China and other countries in the region.
In prepared remarks to foreign ministers at 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.
But Washington continued to urge North Korea to "demonstrate a change in behavior, including ceasing provocative actions," she said, putting the onus on the North for any resumption of six-party talks on its nuclear program.
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 in Bali on Friday. Both emerged saying they hoped to re-start the 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, 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 program in return for diplomatic and economic rewards.
The last round ended in a familiar stalemate in December 2008.
On the other main issue before Asia-Pacific ministers during a week of hectic diplomacy in Bali, Clinton warned all of the rival claimants to the strategically vital East Sea against using force.
She told delegates including Chinese Foreign Minister Yang Jiechi that all sides must respect freedom of navigation and over-flight in the vital trade route, and that the dispute must be resolved according to international law.
Clinton also emphasized that the United States had a "national interest" in the sea's shipping lanes, repeating a line that angered the Chinese at the last ASEAN Regional Forum a year ago in Hanoi.
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 Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagos.
The East Sea is believed to be extremely rich in oil and gas deposits, as well as being home to shipping lanes linking East Asia with Europe and the Middle East.
It has long been considered one of Asia's potential flashpoints, and in recent months the Philippines and Vietnam have expressed anger over what they call China's increasingly aggressive actions in the sea.
China has claimed it wants to resolve the dispute peacefully, while maintaining its historical claim to all of the sea stretching down to the coasts of several Southeast Asian nations.
Clinton and Yang discussed the tensions during bilateral talks on Friday which both sides described as positive. Even so, Yang told Clinton that China's territorial integrity was none of Washington's business, his spokesman said.
China and the 10 members of the Association of Southeast Asian Nations (ASEAN) agreed earlier in the week on a set of guidelines for an eventual code of conduct in the sea.
But the Philippines has complained that the eight-point document lacks teeth, while Clinton called it only an "important first step" towards a final diplomatic solution.