North Korea said on Saturday that it was entering a "state of war" with South Korea, following a call to arms by the country's young leader Kim Jong Un and days of increasingly belligerent rhetoric from the isolated state.
The North's official news agency KCNA published the joint statement issued by the government, political parties and other organizations.
"From this time on, the North-South relations will be entering a state of war and all issues raised between the North and the South will be handled accordingly," it said.
The statement also warned that if the U.S. and South Korea carried out a pre-emptive attack, the conflict "will not be limited to a local war, but develop into an all-out war, a nuclear war."
Analysts have said the North's threats have followed a similar pattern but that the country's 30-year-old leader is unpredictable and potentially dangerous.
The White House responded on Saturday by reiterating that "North Korea has a long history of bellicose rhetoric and threats," National Security Council spokeswoman Caitlin Hayden said in a statement. However, she said the U.S. "takes these threats seriously".
"We continue to take additional measures against the North Korean threat, including our plan to increase the U.S. ground-based interceptors and early warning and tracking radar, and the signing of the ROK-U.S. counter-provocation plan," she said.
On Thursday the U.S. sent two nuclear-capable bombers to South Korea, where they dropped inert munitions in a military exercise. The flight sparked an angry response from the North, which declared on Friday that it was preparing rockets aimed at American bases in South Korea and the Pacific.
A South Korean defense ministry official said there were no early signs that the North was mobilizing, Reuters reported.
The two nations have technically been at war since a truce ended their 1950-53 conflict, but tensions have been increasing since the North carried out its third nuclear weapons test in February.
NBC News' Kristen Welker and Reuters contributed to this report.