The US fears N Korea is developing a long-range missile

"What you've seen is a rather predictable series of escalatory statements from North Korea," said White House spokesman Ari Fleischer.

The grave situation created by the undisguised war acts committed by the US N Korean statement Click for full text

He was speaking after Pyongyang said it would no longer feel obliged to observe the armistice which halted the 1950-53 war in response to what it called US violations of the truce.

Tensions on the peninsula have risen dramatically since North Korea revived its nuclear programme late last year and then last month announced it was pulling out of the nuclear non-proliferation treaty (NPT).

Pyongyang is demanding a non-aggression pact with the US, but Washington refuses to talk until North Korea agrees to shut down its weapons programme.

A US State Department official said North Korea was going about things the wrong way if it wanted direct talks with the US.

"The US will not respond to threats, broken commitments or blackmail by North Korea," he said.

"Any further escalation by North Korea of the situation on the peninsula will bring international condemnation and further self-isolation."

Previous threats

The Korean War ended with an armistice in 1953 - not a peace treaty - and thus the peninsula is technically still at war.

The border between the North and the South is one of the world's most heavily armed.

In theory, if the North withdrew from the armistice it would mean resuming the war, but in practice, it is not clear whether North Korea would actually resume the fighting, says the BBC's Washington correspondent Jon Leyne.

North Korean has previously threatened to pull out of the armistice, in an attempt to force Washington to start negotiations with Pyongyang.

'Act of war'

A statement carried by the official Korean Central News Agency (KCNA) accused the US of bolstering its forces in the region and mounting a naval blockade.

CRISIS CHRONOLOGY 16 Oct: US says N Korea admits to a secret nuclear programme 14 Nov: US halts oil shipments to N Korea 22 Dec: N Korea removes monitoring devices at Yongbyon nuclear plant 31 Dec: UN nuclear inspectors forced to leave 10 Jan: N Korea pulls out of anti-nuclear treaty 12 Feb: IAEA refers issue to UN Security Council Timeline of tensions

It was released after the US and South Korea announced they would stage joint military exercises in the South in March.

The North Koreans said they are responding to recent US actions, including the boarding of a North Korean cargo vessel in December.

The vessel was found by the US to contain Scud missiles, but as they were being legally exported it was eventually allowed to continue.

North Korea also complained of recent US moves to put bombers on standby to move to the region.