U.S. State Secretary Hillary Clinton signs a Treaty of Amity...

U.S. State Secretary Hillary Clinton signs a Treaty of Amity and Cooperation at the end of the 17th ASEAN Regional Forum on Friday in Hanoi. Clinton accused North Korea of a campaign of provocation as an Asia-Pacific security forum descended into recriminations over tensions on the Korean peninsula. (July 23, 2010) Credit: AFP / Getty Images

HANOI - North Korea raised the stakes in its face-off with the United States and South Korea on Saturday, threatening to use nuclear weapons if Washington and Seoul go ahead with military exercises planned for regional waters this summer.

As a U.S. aircraft carrier arrived in South Korea for the air and sea maneuvers, the North told participants in an Asian regional security conference in Hanoi that the move was a threat to its sovereignty and security.

"This is not defensive training," North Korean spokesman Ri Tong Il told reporters.

Hours later, North Korea's powerful National Defense Commission said that "the DPRK will legitimately counter [the drills] with their powerful nuclear deterrence," its state-run news agency reported.

North Korea has conducted two nuclear tests within the past four years and has repeatedly said it will not abandon its nuclear weapons program, despite international efforts.

The exercises were planned in response to the torpedo sinking in March of the South Korean naval ship Cheonan, in which 46 sailors died. An international investigation said North Korea was to blame.

Pyongyang has denied responsibility.

After Secretary of State Hillary Rodham Clinton told participants in the forum here that "an isolated and belligerent North Korea has embarked on a campaign of provocative, dangerous behavior," according to prepared remarks, Ri left the meeting to read a statement. He said the North's "position is clear: There will be a physical response to the threat imposed by the United States military." U.S. officials said afterward that North Korean acts of aggression usually come in a series and that they expected further provocation.

The Obama administration also announced this week a toughening of its economic sanctions against the North.

"Peaceful resolution of the issues on the Korean Peninsula will be possible only if North Korea fundamentally changes its behavior," Clinton said.

Clinton's visit to Asia this week was her fifth as secretary of state as the Obama administration seeks to improve its standing in the region.

The U.S. and South Korean delegations urged the 27-nation Asian Regional Forum to adopt a strong statement condemning the North, but officials from several delegations said that was unlikely. An earlier U.N. Security Council statement was watered down after China said it would withhold its vote.

Clinton also used the conference to buttress Vietnam's position in a dispute with Beijing over the South China Sea, declaring a peaceful resolution of the issue to be in the "national interest" of the United States.

She also lobbied for a multilateral diplomatic solution to the question - an approach Hanoi has been seeking for years. China, which claims most of the sea as its territory, has demanded all disputes be settled bilaterally.

"We oppose the use or threat of force by any claimant," Clinton said in prepared remarks. She later called the conversation "very productive."

Vietnam and the United States were not alone in confronting China on this issue. At least 10 other participants in the meeting raised concerns about maritime security issues, including the sensitive territorial claims in the South China Sea.

Beijing, meanwhile, opposed any effort to "internationalize" the issue.

Clinton has made four stops on her week-long Asian trip, all marking U.S. overseas conflicts.

In South Korea, she and Defense Secretary Robert Gates marked the 60th anniversary of the start of the Korean conflict with a pledge of nearly unlimited support. She was warmly received in Vietnam, where her visit marked the 15th anniversary of the normalization of relations. The war in Afghanistan, where she stopped for a day after visiting Pakistan, has now become the longest in U.S. history.

Correspondent Chico Harlan in Ho Chi Minh City, Vietnam, and staff writer John Pomfret in Washington contributed to this report.

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