North Korea says it has entered a "state of war" with the South and will deal with every inter-Korean issue accordingly.

The announcement was made in a joint statement attributed to all government bodies and institutions and carried by the official Korean Central News Agency (KCNA).

It comes as the North ratchets up talk of striking targets in the United States.

"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 statement said.

"The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over."

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

Earlier this month the North announced 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.

Sorry, this video has expired Former Australian ambassador to South Korea, Mack Williams, discusses the situation

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

Voiding the ceasefire theoretically opens the way to a resumption of hostilities, although observers note it is far from the first time that North Korea has 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.

Today's statement also warns 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.

Sorry, this video has expired Mark Willacy reports on North Korea's threats against the US

Yesterday the North issued a statement saying it had put its rockets "on standby" to strike the US mainland and military bases.

Leader Kim Jong-Un vowed to "settle accounts" after nuclear-capable US stealth B-2 bombers flew over South Korea for a drill on Thursday.

The bulk of the threats emanating from Pyongyang have been dismissed as bluster.

North Korea has no confirmed missile capability to reach the US mainland - or indeed Guam or Hawaii in the Pacific.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 41 seconds 3 m 41 s Tension rising between North Korea and US ( Jane Cowan ) Download 1.7 MB

But Washington has opted to match the threats with its own muscle-flexing.

"We've seen reports of a new and unconstructive statement from North Korea," said Caitlin Hayden, spokeswoman for the National Security Council.

"We take these threats seriously and remain in close contact with our South Korean allies.

"We would also note that North Korea has a long history of bellicose rhetoric and threats and today's announcement follows that familiar pattern.

"We continue to take additional measures against the North Korean threat, including our plan to increase the US ground-based interceptors and early warning and tracking radar."

America's stealth bomber flights, which followed training runs by B-52 bombers, were part of annual drills between the United States and South Korea, which North Korea each year denounces as rehearsals for war.

Pyongyang has been particularly vocal this time, angered by UN sanctions imposed after its long-range rocket launch in December and the third nuclear test it carried out last month.

Sorry, this video has expired Chuck Hagel says US is taking threats seriously

ABC/AFP