N. KOREA THREATENS MILITARY ACTION

North Korea's military celebrated the controversial nuclear and missile tests

North Korea has warned of a military response after South Korea joined an anti-proliferation exercise which could allow it to search the North's ships.

The North said it is no longer bound by the armistice which ended the Korean War in 1953.

A military spokesman quoted by official media said Pyongyang could no longer guarantee the safety of shipping.

Its latest threat comes after an underground nuclear test two days ago and several missile launches.

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

Meanwhile steam is reported to be coming from North Korea's main nuclear reprocessing plant at Yongbyon, suggesting it has made good on its threat to restart efforts to make weapons-grade plutonium.

South Korea announced on Tuesday that it would not delay any longer in joining the Proliferation Security Initiative (PSI) - a US-led non-proliferation campaign involving searching ships carrying suspicious cargos and aimed at stopping the trafficking of weapons of mass destruction.

Your views: Should world worry? Reaction from around the region What is N Korea's game plan? Factfile: Underground testing

North Korea has repeatedly warned that the South's participation in the PSI would be tantamount to a declaration of war.

Joining the PSI "is a natural obligation", South Korean Foreign Minister Yu Myung-hwan said, quoted by South Korea's Yonhap news agency. "It will help control North Korea's development of dangerous material."

North Korea's response has been firm.

"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," a spokesman for the North's army was quoted as saying by the official KCNA news agency.

North Korea has fired five short-range missiles in two days, despite strong censure from the international community, including China and Russia.

NUCLEAR CRISIS Oct 2006 - North Korea conducts an underground nuclear test Feb 2007 - North Korea agrees to close its main nuclear reactor in exchange for fuel aid June 2007 - North Korea shuts its main Yongbyon reactor June 2008 - North Korea makes its long-awaited declaration of nuclear assets Oct 2008 - The US removes North Korea from its list of countries which sponsor terrorism Dec 2008 - Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aid Jan 2009 - The North says it is scrapping all military and political deals with the South, accusing it of "hostile intent" April 2009 - Pyongyang launches a rocket carrying what it says is a communications satellite 25 May 2009 - North Korea conducts a second nuclear test Q&A: North Korea nuclear test What are N Korea's motives?

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

That threat last month was prompted, it said, by the UN Security Council's censure of North Korea's 5 April rocket launch.

Washington is calling for a quick and unified response by the international community that will make it clear to North Korea that there are consequences for its actions.

Diplomats from the five permanent Security Council member countries plus Japan and South Korea have been meeting behind closed doors to discuss a new resolution.

"We are thinking through complicated issues that require very careful consideration," said the US ambassador to the UN, Susan Rice.

US State Department spokesman Ian Kelly also said the door was still open to resume long-running six-party talks and that the US was looking at a "whole range of options".

It is a sign of the delicate balance required to handle the reclusive country, the BBC's State Department correspondent Kim Ghattas reports from Washington.

China shares a border with North Korea and worries about pushing Pyongyang too far, so it is unclear what sort of measures might be taken now and how North Korea would respond, our correspondent adds.

This week's test and missile launches came after North Korea walked away from long-running disarmament talks.

It agreed in February 2007 to abandon its nuclear ambitions in return for aid and diplomatic concessions.

But the negotiations stalled as it accused its negotiating partners - the US, South Korea, Japan, China and Russia - of failing to meet agreed obligations.

BBC NEWS REPORT.