North Korea on Saturday declared it was in a "state of war" with South Korea and warned Seoul and Washington that any provocation would swiftly escalate into an all-out nuclear conflict.

The United States said it took the announcement "seriously", but noted it followed a familiar pattern, while South Korea largely dismissed it as an old threat dressed in new clothing.

It was the latest in a string of dire-sounding pronouncements from Pyongyang that have been matched by tough warnings from Seoul and Washington, fuelling international concern that the situation might spiral out of control.

"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 government statement carried by the Korean Central News Agency.

"The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over," the statement said, adding that any US or South Korean provocation would trigger a "a nuclear war".

The two Koreas have technically remained at war for the past six decades 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.

The White House labelled the latest statement from Pyongyang as "unconstructive" and, while taking it "seriously", sought to place the immediate threat level in context.

"North Korea has a long history of bellicose rhetoric and threats and today's announcement follows that familiar pattern," said National Security Council spokeswoman Caitlin Hayden.

In Seoul, the Unification Ministry insisted the war threat was "not really new". The Defence Ministry vowed to "retaliate thoroughly" to any provocation, but added that no notable troop movement had been observed along the border.

Late on Saturday the North ratcheted up its threats, warning it could shut down the Kaesong industrial complex, a joint South-North venture that provides the regime with crucial hard currency.

"We warn that stern measures will be taken if (South Korea) continues to make reckless remarks defaming our dignity," a state body said, adding that the fate of the complex depends entirely on the attitude of Seoul.

Established in 2004 as a symbol of cross-border cooperation, Kaesong had managed to keep functioning despite repeated crises in inter-Korean relations.

But there have been concerns that its operations would be affected by Pyongyang's move on Wednesday to sever a military hotline used to monitor movement in and out of the zone.

Most observers however still believe this will remain a verbal rather than a physical battle.

"The North Koreans in recent weeks have turned rhetoric into performance art," said Gordon Flake, a Korea specialist and executive director of the Mansfield Foundation in Washington.

"When they have already declared the armistice null and void, I do not think a declaration of war breaks new ground," Flake said.

But he added that the situation had now become so volatile that any slight miscalculation carried the potential for rapid escalation.

Both China and Russia have called for calm, with a Russian diplomat voicing particular concern on Saturday.

"We expect all sides to show maximum responsibility and restraint and that no-one will cross the line after which there will be no return," Grigory Logvinov, a Russian foreign ministry pointman on North Korea, told the Interfax news agency.

France on Saturday urged North Korea to refrain from any new provocation and return on the path of dialogue. Meanwhile Britain said threatening statements would only further isolate Pyongyang.

On Friday North Korean leader Kim Jong-Un ordered missile units to prepare to strike US mainland and military bases, after US stealth bombers flew over South Korea.

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.

Military balance on the Korean peninsula
Seoul (AFP) March 31, 2013 - Following North Korea's declaration that it is in a "state of war" with South Korea, here is a fact box on the relative conventional strengths of the three militaries present on the Korean peninsula.

The figures are taken from the London-based International Institute of Strategic Studies' annual global "Military Balance" report, 2011.

North Korea's annual defence spending as of 2008 was estimated at $8.2 billion, or 22-24 percent of GDP, while South Korea's as of 2012 was $30.8 billion, or 2.7 percent of GDP.

South Korea is protected by the US "nuclear" umbrella, while North Korea, which conducted its third nuclear test in February, claims a potent nuclear weapons capability.

The range of North Korean missiles, and its ability to manufacture and deliver working nuclear warheads, are a matter of dispute.

                               North Korea          South Korea (+ US forces)

GROUND FORCES

   Active troops                  1.2 million          655,000 (+ 28,000)
   Reserves/Paramilitaries        5-7.7 million        3.0 million
   Tanks                          4,100                2,400 (+ 50)
   Armoured personnel carriers    2,500                2,600 (+ 110)
   Field artillery pieces         8,500                5,200 (+ 16)
   Multiple rocket launchers      5,100                200 (+ 40)
   Mortars                        7,500                6,000
   Air Defence Guns               11,000               300

AIR FORCES

   Combat aircraft                820 (620 serviceable) 460 (+ 90)
   Helicopters                    300                   680 (+ 120)

NAVAL FORCES

   Principle combat vessels       3                     19 
   Patrol and coastal vessels     383                   111
   Submarines                     70                    23
   Hovercraft                     135                   5
   Landing ships and craft        130                   41