North Korea has threatened to attack the South following checks for weapons of mass destruction on its naval fleet.

It comes after Seoul joined a US-led initiative to check vessels suspected of carrying nuclear equipment.

Pyonyang was already facing international condemnation for its nuclear test at the weekend and subsequent missile launches, but this latest development is a major escalation in tensions.

Downing Street expressed "concern" about the latest developments.

"This is clearly a further unnecessary and provocative act by North Korea which will do nothing to advance the prospects for regional security and will only serve to isolate the regime further," Prime Minister Gordon Brown's spokesman said.

A North Korean army spokesman also said the country was not obliged to maintain the armistice signed at the end of the 1950-53 Korean War because Washington had ignored its responsibility as a signatory by drawing South Korea into its naval initiative.

A spokesman for the North's army 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."

South Korea had confirmed it was joining the naval exercise, called the Proliferation Security Initiative.

It comes after South Korean media reported that Pyongyang had restarted the Yongbyon plant that makes weapons-grade plutonium.

Pyongyang also appeared to have fired a third short-range missile after it added to tensions with a launch of two others.

© Independent Television News Limited 2009. All rights reserved.