WASHINGTON (AFP) - The White House said it took a new warning from North Korea that it was in a state of war with South Korea seriously, but said Pyongyang's threats were following a familiar pattern.

"We've seen reports of a new and unconstructive statement from North Korea," said Ms Caitlin Hayden, spokeswoman for the National Security Council on Friday.

"We take these threats seriously and remain in close contact with our South Korean allies."

North Korea earlier delivered the latest in a string of dire threats that have drawn a tough response from Seoul and Washington, and sparked international concern that tensions on the peninsula could get out of control.

Pyongyang said it had formally entered a "state of war" with South Korea and warned that any provocation would swiftly escalate into a nuclear conflict.

While seeking to show appropriate concern at Pyongyang's latest statement, Washington also sought to place the new North Korean threat into the context of a string of rhetoric from Kim Jong-Un's regime.

"We would also note that North Korea has a long history of bellicose rhetoric and threats and today's announcement follows that familiar pattern," Ms Hayden said, noting that the United States was fully capable of protecting itself and its Asian 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," she said.

Earlier, deputy White House spokesman Josh Earnest told reporters on Air Force One that Pyongyang was purely to blame for escalating tensions.

"We are coordinating pretty closely with not just our allies, but also with Russia and China which also have a significant stake in resolving this situation peacefully," Mr Earnest said.

"The path to peace for the North Koreans is clear," namely halting bellicose rhetoric, ending the nuclear programme and putting their people first.