N Korea threatens military action
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 two days of underground nuclear tests 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 conventions.
Anti proliferation
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.
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.
Reactivated reprocessing?
North Korea has fired five short-range missiles in two days, despite strong censure from the international community, including China and Russia.
Q&A: North Korea nuclear test
What are N Korea's motives?
Meanwhile South Korean media reported that steam was seen 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 coming from it," an unidentified official told the Chosun Ilbo newspaper. Similar reports were carried by the Yonhap news agency.
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.
Censure dilemmas
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.