South Korean and US troops were put on higher alert Thursday after North Korea announced it is scrapping the armistice that has been in force on the peninsula for more than five decades.

"As of 7:15 am Thursday (2215 GMT Wednesday), the US-South Korea Combined Forces Command upgraded Watch Conditions by a notch to Stage Two," Seoul's defence ministry said in a statement.

"Surveillance over the North will be stepped up, with more aircraft and personnel mobilised," said spokesman Won Tae-Jae.

North Korea announced Wednesday it was abandoning the armistice that ended the 1950-53 Korean War and warned it could launch a military attack on the South, two days after testing an atomic bomb for the second time.

Its latest display of anger was prompted by the South's decision to join a US-led international initiative to stop the spread of weapons of mass destruction.

The Proliferation Security Initiative can involve stopping and searching ships.

The North said its military would no longer be bound by the truce that ended hostilities in the war, in which the United States led a United Nations force defending the South.

"Any tiny hostile acts against our republic, including the stopping and searching of our peaceful vessels... will face an immediate and strong military strike in response," its military said.

The United States still has 28,500 troops stationed in South Korea as a deterrent against the North.

Won said it was the fourth time since 1982 that the watch conditions (Watchcon) had been upgraded to stage two. The last time was in October 2006, after the North's first nuclear test.

He said surveillance would be focused along the Demilitarised Zone which divides the peninsula, the Joint Security Area at Panmunjom village and the Northern Limit Line - the disputed border in the Yellow Sea.

"We are maintaining a tight defence posture to prevent the North's military provocations," Won said. "The military will deal sternly with provocative acts."

The North has previously threatened to abandon the truce.

"This is the fifth time in 15 years that they've sought to nullify the armistice governing the Korean War," said White House spokesman Robert Gibbs in Washington. "I think their actions would be better focused on living up to their rights and obligations."

The US-led UN Command said it remains committed to the armistice, which "has served as the legal basis for the ceasefire in Korea for over 55 years and significantly contributes to stability in the region.

"The armistice remains in force and is binding on all signatories, including North Korea," it said in a statement. "The UN Command will adhere to the terms of the armistice and the mechanisms that support it."