The White House said Saturday it is taking seriously new threats by North Korea but also noted Pyongyang's history of "bellicose rhetoric."

North Korea warned Seoul on Saturday that the Korean Peninsula had entered "a state of war." It also threatened to shut down a border factory complex that is the last major symbol of co-operation between the Koreas.

"We've seen reports of a new and unconstructive statement from North Korea. We take these threats seriously and remain in close contact with our South Korean allies," said Caitlin Hayden, a spokeswoman for the White House National Security Council. "But, we would also note that North Korea has a long history of bellicose rhetoric and threats, and today's announcement follows that familiar pattern."

North Korea's threats are seen as part of an effort to provoke the new government in Seoul to change its policies toward Pyongyang, and to win diplomatic talks with Washington that could get it more aid. The moves also are seen as ways to build domestic unity as North Korea's young leader, Kim Jong Un, strengthens his military credentials.

In recent days, the U.S. flew a pair of nuclear-capable B-2 stealth bombers over an uninhabited South Korean island, dropping dummy munitions as part of annual defence drills that North Korea views as rehearsals for an invasion. Defence Secretary Chuck Hagel also announced that the U.S. will fortify its defences against a potential North Korean missile attack on the U.S. by adding more than a dozen missile interceptors to the 26 already in place at Fort Greely, Alaska.

'Wartime regulations'

North Korea said in a statement Saturday that it would deal with South Korea according to "wartime regulations" and would retaliate against any provocations by the U.S. and South Korea without notice.

"Now that the revolutionary armed forces of the DPRK have entered into an actual military action, the inter-Korean relations have naturally entered the state of war," said the statement, which was carried by the official North Korean news agency and referred to the country by its official name, the Democratic People's Republic of Korea.

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Provocations "will not be limited to a local war, but develop into an all-out war, a nuclear war," the statement said.

The White House has stressed the U.S. government's capability and willingness to defend itself and its allies and interests in the region, if necessary.

"We remain fully prepared and capable of defending and protecting the United States and our allies," Hayden said.