The US has said it is in close touch with South Korea and is prepared for any possible threat from North Korea, amid escalating tension in the Korean peninsula after declaration of "state of war" by Pyongyang.

"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, spokesperson of the 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," she said.

Referring to the statement by Defense Secretary Chuck Hagel, Hayden said the US remains fully prepared and capable of protecting the country and its allies.

"We continue to take additional measures against the North Korean threat, including our plan to increase the US ground-based interceptors and early warning and tracking radar, and the signing of the ROK-US counter-provocation plan," Hayden said.

The White House statement came after North Korea yesterday said it was entering a "state of war" with South and warned that US bases in Hawaii and Guam would be targeted in what could turn into "an all-out war, a nuclear war."

According to North Korea's official news agency, its leader Kim Jong Un, after his meeting with his top commanders, said his country "would react to the US nuclear blackmail with a merciless nuclear attack, and war of aggression with an all-out war of justice."

Kim also signed the plan on technical preparations of strategic rockets, ordering them to be "standby for fire so that they may strike any time the US mainland, its military bases in the operational theaters" in the Pacific, including Hawaii and Guam, and those in south Korea.