North Korea has threatened to pull out of the armistice agreement that ended the Korean War, accusing the United States of breaking its terms.

CRISIS CHRONOLOGY

16 Oct: US announces that N Korea has acknowledged 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 North Korea 10 Jan: N Korea pulls out of anti-nuclear treaty 28 Jan: President Bush urges the "oppressive" N Korean regime to give up its nuclear ambitions 12 Feb: IAEA refers issue to Security Council

Timeline of tensions

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

Earlier on Monday, the United States and South Korea announced they would stage joint military exercises in South Korea next month, in a move which was thought likely to heighten tensions with the North.

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

The Korean War ended with an armistice in 1953 - not a peace treaty.

This means that the peninsula is still technically 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'

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 Americans to contain Scud missiles, but as they were being legally exported it was eventually allowed to continue.

The grave situation created by the undisguised war acts committed by the US N Korean statement

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

Tensions escalated in October, when the US accused Pyongyang of running a secret nuclear weapons program in violation of a 1994 agreement.

Under the agreement, the United States provided heavy fuel oil, while North Korea halted its nuclear programme.

After the shipments were suspended, North Korea reactivated a nuclear facility capable of producing plutonium, saying it needed to make up the shortfall in energy supplies.

The North Koreans are believed to possess one or two nuclear weapons already, as well as enough spent fuel rods to make four or six more.

The UN Security Council is due to discuss the crisis soon - a move the Koreans have fiercely condemned.

Pyongyang has said the imposing of sanctions would be seen as an act of war.