North Korea has threatened to attack South Korea after it joined a US-led initiative to check vessels suspected of carrying equipment for weapons.
The initiative, which includes military drills, was launched by then-US President George W Bush in 2003.
A North Korean army spokesman also said the country was no longer bound by the armistice signed at the end of the 1950-53 Korean War because the US had ignored its responsibility as a signatory by drawing South Korea into its naval initiative.
The threat comes after South Korean media reported earlier that Pyongyang had restarted a plant that makes weapons-grade plutonium.
'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 spokesman for the North's army was quoted as saying by the official KCNA news agency.
South Korea announced yesterday that it was joining the naval exercise, called the Proliferation Security Initiative.
North Korea also appeared to have test-fired a third short-range missile late last night after it added to tension with a launch of two others earlier in the day.
The launch came as the UN Security Council met to discuss ways of solving the situation.
North Korea now appears to have restarted the facility at its Yongbyon nuclear plant that extracts plutonium.
Until recently, the Soviet-era plant was being taken apart under a six-country disarmament-for-aid deal.
'There are various indications that reprocessing facilities in Yongbyon resumed operation (and) have been detected by US surveillance satellite, and these including steam coming out of the facility,' it quoted an unnamed government source as saying.
However there are no signs yet that North Korea, which conducted its only prior nuclear test in October 2006, is again separating plutonium.
