The Pyongyang regime said it has ended the truce with South Korea that ended the Korean War, the BBC reported.
Caitlin Hayden, a spokeswoman for the National Security Council, said the U.S. government takes the threat seriously while recognizing North Korea has a long history of bellicose statements, The Hill reported.
"We take these threats seriously and remain in close contact with our South Korean Allies," Hayden said. "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."
It was the latest in a series of threats from Pyongyang since a third nuclear test Feb. 12 led to the imposition of new international sanctions.
"From this time on, the North-South relations will be entering the state of war and all issues raised between the North and South will be handled accordingly," the North Korean statement said. "The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over."
The two Koreas have been in a technical truce since the end of the Korean War. No peace treaty was ever signed.
The South Korean defense ministry denounced the threats, adding in a statement: "Our military is maintaining full preparedness to leave no blind point in safeguarding the lives and safety of the people," Yonhap News Agency reported.