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

"As of now, inter-Korea relations enter a state of war and all matters between the two Koreas will be handled according to wartime protocol," the North said in a statement attributed to all government bodies and institutions.

It is the latest in a string of dire threats from Pyongyang that have been matched by tough warnings from South Korea and the United States, fuelling international concerns that the situation is spiralling out of control.

"The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over," said the statement carried by the official Korean Central News Agency (KCNA).

The two Koreas have always technically remained at war because the 1950-53 Korean War concluded with an armistice rather than a peace treaty.

The North had announced earlier this month that it was ripping up the armistice and other bilateral peace pacts signed with Seoul in protest against South Korea-US joint military exercises.

"This is not really a new threat -- just part of a series of provocative threats," the South's Unification Ministry said in a statement.

The defence ministry added that no particular troop movement had been observed along the border.

Voiding the ceasefire theoretically opened the way to a resumption of hostilities, although observers noted it was far from the first time that North Korea had announced the demise of the armistice.

The armistice was approved by the UN General Assembly, and both the United Nations and South Korea have repudiated the North's unilateral withdrawal.

Saturday's statement warned that any military provocation near the North-South land or sea border would result "in a full-scale conflict and a nuclear war".

Most observers still believe this will remain a rhetorical rather than a physical battle, but the situation has now become so volatile that any slight miscalculation carries the potential for rapid escalation.

Both China and Russia asked for all sides to cooperate to prevent the situation worsening on Friday, with Russian Foreign Minister Sergei Lavrov voicing particular concern.

"We can simply see the situation getting out of control, it would spiral down into a vicious circle," Lavrov told reporters at a news conference.

His warning came after North Korean leader Kim Jong-Un ordered missile units to prepare to strike US mainland and military bases, vowing to "settle accounts" after US stealth bombers flew over South Korea.

US Defense Secretary Chuck Hagel stressed that Washington would not be cowed by Pyongyang's bellicose threats and stood ready to respond to "any eventuality".

The high-stakes standoff has its roots in North Korea's successful long-range rocket launch in December and the third nuclear test it carried out in February.

Both events drew UN sanctions that incensed Pyongyang, which then switched the focus of its anger to the annual joint South Korea-US military drills.

As tensions escalated, Washington has maintained a notably assertive stance, publicising its use of nuclear-capable B-52s and B-2 stealth bombers in the war games.

The long-distance deployment of both sets of aircraft out of bases in Guam and the US mainland were intended as a clear signal of US commitment to defending South Korea against any act of aggression.

While Hagel stressed that Washington was taking the threats from the North "very seriously," US military intelligence noted that Pyongyang's combative rhetoric has not, so far, been matched by any overtly provocative troop build-up.

And analysts have underlined that the threats and counter-threats have increasingly stressed a conditional element of the other side acting first.