A North Korean spokesman says Pyongyang is severing all relations and cutting communications links with South Korea. North Korea says the move is punishment for what it calls a "smear campaign" over the sinking of a South Korean warship. Tensions are rapidly rising on the Korean peninsula.
North Korean official media accused South Korea Tuesday of trespassing in its territorial waters, and warned of military action.
Pyongyang says "dozens of warships" have entered waters west of the Korean peninsula in the past 10 days, and warns it will "put into force practical military measures to defend its waters."
North Korea has disputed a maritime border in the region for decades.
The Seoul government has accused North Korea of sinking a South Korean navy ship near the border and on Monday, President Lee Myung-bak announced package of retaliatory measures, such as cutting trade.
Repeating a warning he made in a televised address, Mr. Lee warned the situation is serious.
Lee says his country is in a dire situation.
Security concerns have spiked since last week, when a multinational investigation team revealed extensive evidence linking North Korea to the patrol ship sinking. Forty-six South Korean sailors were killed in the incident.
Seo Jae Pyung is director of North Korea Intellectuals Solidarity, a group of North Korean defectors based in Seoul. He says his contacts in the North report that the government is preparing them for conflict.
He says the vice chairman of North Korea's powerful National Defense Commission read an official statement on domestic radio warning the army and related organizations to prepare thoroughly for war.
A separate North Korean news announcement warns if the South attacks the North, "our military and people will not lose the splendid opportunity to reunify the nation."
South Korea and the United States are planning to hold anti-submarine naval drills in the vicinity of the patrol ship sinking soon. A spokeswoman for the Lee administration says South Korea is considering re-designating North Korea as the country's "main enemy" in an annual defense brief. That designation was dropped six years ago, when South Korea was softening its tone as part of a policy to engage the North.
U.S. Secretary of State Hillary Clinton is due in Seoul Wednesday for meetings with President Lee and other top officials. She is working with Seoul to build support for tough new sanctions against the North at the United Nations Security Council.
Those efforts are seen to hinge on China, which is a permanent member of the council, but has been reluctant to do anything that might destabilize North Korea. Wu Dawei, China's main delegate to multinational talks on North Korea's nuclear weapons, was in Seoul Tuesday but did not comment on the current situation. A foreign ministry spokesman in Beijing called for "restraint" on all sides.