The United States launched a joint naval drill Thursday with South Korea and Japan despite dire warnings of a "horrible disaster" from North Korea, AFP reports citing officials.
The two-day drill involving a US nuclear-powered aircraft carrier alongside South Korean and Japanese vessels began off the southern coast of the Korean peninsula, the South's defence ministry said.
The drill comes after Seoul and Washington last week agreed a joint strategy to address what they described as the mounting threat of a North Korean nuclear attack after the communist country restarted an ageing plutonium reactor.
The annual drill -- which officials describe as a search and rescue exercise to improve readiness for humanitarian disasters -- had been scheduled to take place earlier this week, but was postponed due to an approaching typhoon.
"This annual training is aimed at carrying out joint maritime search and rescue operations for humanitarian purposes," Wi Wong-Seop, a South Korean defence ministry spokesman, told reporters.
Besides the nuclear-powered carrier USS George Washington, guided-missile ships, anti-submarine helicopters and early warning aircraft have also been mobilised for the exercise, Yonhap news agency said.
North Korea has habitually condemned joint military drills south of the border and threatened counter-attacks that have not materialised.
On Tuesday North Korea warned the United States of a "horrible disaster" and put its troops on alert.
North Korean troops had been ordered to "keep themselves fully ready to promptly launch operations any time", a spokesman for the General Staff of the Korean People's Army said in a statement.
"The US will be wholly accountable for the unexpected horrible disaster to be met by its imperialist aggression forces," the statement said.
Analysts have attributed the isolated regime's recent bellicose rhetoric to its desire to attract the United States' attention and draw it back into dialogue.
The United States and South Korea have long demanded that Pyongyang show commitment to ending its nuclear weapons programme before six-nation talks on the programme, which have been stalled since December 2008, can resume.
Although the North's atomic test in February -- its most powerful to date -- sent tensions soaring, the temperature has been lowered in recent months after a series of conciliatory gestures by Pyongyang towards Seoul.
But acute concerns remain over the North's nuclear programme, with a US think tank saying last week that Pyongyang has restarted its ageing Yongbyon reactor.