North Korea warned on Wednesday of a possible military response after South Korea joined an anti-proliferation exercise, and said it is no longer bound by the 1953 armistice which ended their war.
A military statement quoted by official media also said the North could no longer guarantee the safety of shipping off its west coast.
It repeated Pyongyang's position that Seoul's decision to join the US-led Proliferation Security Initiative (PSI) is tantamount to a declaration of war.
"Any tiny hostile acts against our republic, including the stopping and searching of our peaceful vessels... will face an immediate and strong military strike in response," the statement said.
"Our military will no longer be bound by the armistice accord as the current US leadersship... has drawn the puppets (South Korea) into the PSI," said the statement from the North's military representative at the border truce village of Panmunjom.