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 -- to condemn the act but not specifically blame North Korea -- after China said it would withhold its vote.
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.