North Korea has said it is entering a "state of war" with South Korea in its latest escalation of rhetoric against its southern neighbour and the US.
A statement promised "stern physical actions" against "any provocative act".
North Korea has threatened attacks almost daily after it was sanctioned for a third nuclear test in February. It has also reacted angrily to annual US-South Korean military exercises.
However, few think the North would risk a full-blown war.
North and South Korea have technically been at war since the armed conflict between them ended in 1953, because an armistice was never turned into a peace treaty.
The North carried out its third nuclear test on 12 February, which led to the imposition of fresh sanctions.
Many analysts believe that all-out war with South Korea and its ally the United States would be suicidal for Pyongyang, says the BBC's Lucy Williamson in Seoul.,
But with both sides threatening heavy retaliation, there's a chance of minor incidents escalating, our correspondent adds.
'Taking threats seriously'
A North Korean statement released on Saturday said: "From this time on, the North-South relations will be entering the state of war and all issues raised between the North and the South will be handled accordingly."
"The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over."
In Washington Caitlin Hayden, a spokeswoman for the National Security Council, said the US had "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," she said.
North Korea has made multiple threats against both the US and South Korea in recent weeks, including warning of a "pre-emptive nuclear strike" on the US and the scrapping of the Korean War armistice.
On Thursday, North Korean state media reported leader Kim Jong-un "judged the time has come to settle accounts with the US imperialists".
He was said to have condemned US B-2 bomber sorties over South Korea as a "reckless phase" that represented an "ultimatum that they will ignite a nuclear war at any cost on the Korean Peninsula".
US mainland and bases in Hawaii, Guam and South Korea were all named as potential targets.
State media in the North showed thousands of soldiers and students at a mass rally in Pyongyang supporting of Kim Jong-un's announcement
North Korea's most advanced missiles are thought to be able to reach Alaska, but not the rest of the US mainland.
The US had already flown nuclear-capable B-52 bombers over South Korea earlier this month, in what it called a response to escalating North Korean threats.
White House spokesman Josh Earnest said the rhetoric only deepened North Korea's isolation.
China, North Korea's biggest trading partner, has reiterated its call for all sides to ease tensions.
When you look at occasions where something really did happen, such as the artillery attack on a South Korean island in 2010, you see there were very clear warnings Professor John Delury, Yonsei university ”
Foreign Ministry spokesman Hong Lei told a daily news conference that "joint efforts" should be made to turn around a "tense situation".
Russian Foreign Minister Lavrov went further, voicing concern that "we may simply let the situation slip out of our control and it will slide into a spiral of a vicious circle".
"We are concerned that… unilateral action is being taken around North Korea that is increasing military activity," he said.
On 16 March, North Korea warned of attacks against South Korea's border islands, and advised residents to leave the islands.
In 2010 it shelled South Korea's Yeonpyeong island, causing four deaths.
On Wednesday, Pyongyang also cut a military hotline with the South – the last direct official link between the two nations.
A Red Cross hotline and another line used to communicate with the UN Command at Panmunjom have already been cut, although an inter-Korean air-traffic hotline still exists.
The jointly run Kaesong industrial park is still in operation.