WORLD: 1953 truce not valid, says Korea

The latest act of aggression came in response to South Korea joining an anti-proliferation exercise which could allow it to search the North's ships, a move aimed at stemming the flow of weapons of mass destruction into and out of the nuclear state.

North Korea said it could no longer guarantee the safety of shipping and that the South's participation in the exercise would be tantamount to a declaration of war. It threatened military action if its ships were searched.

North Korea has restarted a weapons-grade nuclear plant, it was reported, while it staged a mass rally to celebrate its second nuclear test.

South Korea joined the American-led "proliferation security initiative" after Monday's underground nuclear test. Foreign minister Yu Myung-hwan called the naval exercise "a natural obligation", saying: "It will help control North Korea's development of dangerous material."

A North Korean military spokesman said: "Any hostile act against our peaceful vessels including search and seizure will be considered an unpardonable infringement on our sovereignty and we will immediately respond with a powerful military strike."

The isolated communist regime has launched missiles as well as its nuclear test which seemed designed to inflame global opinion. North Korea has fired five short-range missiles in two days, despite international censure, including from China and Russia.

The UN Security Council is working on a strong condemnation of what it says is North Korea's contravention of its conventions.

South Korean media has reported that steam was coming from North Korea's nuclear plant at Yongbyon, suggesting the fuel reprocessing plant there had been reactivated.

"US spy satellites recently spotted various signs of the once-frozen reprocessing facility being reactivated, such as water vapour," an official told the Chosun Ilbo newspaper.

The North announced last month it was quitting a six-nation nuclear disarmament pact and would reopen the Yongbyon plant, closed in July 2007.

Thousands of Pyongyang residents, including senior military and party officials, ignored global opinion yesterday as they gathered in a stadium to celebrate the nuclear test.