WASHINGTON (REUTERS) - The White House said on Saturday that it takes North Korea's latest saber-rattling threats seriously while cautioning that Pyongyang has a long history of bellicose rhetoric.

North Korea's latest bout of angry rhetoric included a vow that it is entering a "state of war" with South Korea a day after North Korean leader Kim Jong Un signed off on an order putting its missile units on standby to attack US military bases in the South.

"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 Ms Caitlin Hayden, 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," she said.

The United States flew two nuclear-capable stealth bombers over the Korean peninsula as part of a military exercise this week in a show of force to reassure US allies in the region.

The Pentagon has also been beefing up US missile defence capabilities on the West Coast. The United States has been stressing that it has the capability and willingness to protect itself and US allies in the region.

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

"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 recent signing of a South Korean-US counter-provocation plan, she added.