The action comes a day after South Korea joined a US-led initiative to intercept shipments suspected of being involved in weapons of mass destruction.
The communist North's military said it will respond with "immediate, strong military measures" if the South stops and searches any North Korean ships under the Proliferation Security Initiative.

"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.