North Korea on Saturday blasted South Korea for hosting a multi-national naval drill aimed at preventing the transfer of weapons of mass destruction, calling it an "open declaration of war".
South Korea for the first time played a full part in the Proliferation Security Initiative (PSI) on Thursday, with warships and aircraft from four countries staging an exercise off the southern port of Busan.
The reclusive North, where leader Kim Jong-Il is paving the way for his son Kim Jong-Un to succeed him, is nuclear-armed.
"The (South Korean) puppet regime revealed its criminal plot to ruin the hard-won atmosphere for dialogue and peace and to drive inter-Korean ties to the edge of a war by hosting the PSI," the North's Minju Joson daily said.
"This is an outright military provocation and an open declaration of war against us," the official daily of the North's government said in a commentary, according to Pyongyang's Korean Central News Agency.
In a separate commentary the North's ruling communist party's official daily, Rodong Sinmun, said the country would build up its armed forces 1,000-fold should the United States continue its military threat against it.
The North already has one of the largest standing armies in the world.
The South says the drill does not target specific countries but the Minju Joson said the exercise was aimed at interdicting and searching ships that belong to the communist state.
About 10 warships and aircraft from South Korea, the United States, Japan and Australia took part in the one-day manoeuvres.
"A naval blockade, which can be seen only during wartime, must not be tolerated," it said, adding the exercise was aimed at "seizing, inspecting and searching our ships and blockading our ports."
"Illegal provocative acts such as the PSI would bring nothing but military conflicts and worse inter-Korean relations," it warned.
South Korea was previously only an observer in the initiative for fear of offending its communist neighbour. After the North's second nuclear test in May last year, it said it would become a full member.
Seoul announced it would host the latest exercise after accusing Pyongyang of torpedoing one of its warships in March and killing 46 sailors.