The United States and South Korea have decided to stage joint military exercises in South Korea next month, in a move which is likely to heighten tensions with the North.

US officials insisted that the exercises were a planned, annual event, unrelated to a stand-off over North Korea's nuclear ambitions.

But they are likely to infuriate the North, which has already warned the US against any military build-up in the region.

The US is also reported to have agreed with its other regional ally, Japan, to conduct joint tests designed to intercept ballistic missiles, a Japanese newspaper said on Monday.

Both moves came in the wake of last week's decision by the United Nations' nuclear watchdog to refer Pyongyang to the UN Security Council for violating non-proliferation accords after it took steps to restart its controversial nuclear programme.

The US fears N Korea is developing a missile capable of reaching the western US

The council has the power to impose sanctions on North Korea - a move Pyongyang said it would regard as a declaration of war.

The dispute continued over the weekend, with Pyongyang insisting it would win any show-down because of its "ultra-hardline" military stance.

North Korea has said the only way out of the impasse is through direct talks with Washington, but on Sunday, US national security adviser Condoleezza Rice reiterated that the US would only support a multilateral approach.

US officials quoted in The New York Times on Monday said Washington was already developing plans for sanctions against Pyongyang, even though the US said last week it would not immediately push for sanctions.

Within North Korea itself, the weekend's events mainly centred around celebrations to mark the 61st birthday of the country's autocratic leader, Kim Jong-il.

The celebrations were held despite a serious economic crisis, in which many people are said to have already starved to death.

Military exercises

The US-South Korea military drills, due to start on 4 March, are designed to improve South Korea's defence capabilities against "external aggression", the US military said on Monday.

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

The exercises will focus on a mock battle, and are aimed at training US forces despatched to Korea in case of emergency.

A total of 5,000 American soldiers from the US and the Pacific region will participate in the drills, a Combined Forces Command (CFC) official said on Monday.

The Japanese-US missile tests are due to start in April 2004 in Hawaii, according to the Nihon Keizai Shimbun newspaper.

Tokyo began studying interception technology with the US after North Korea test-fired a suspected Taepodong-1 missile that passed over Japan in August 1998.

Many countries in the East Asian region are concerned about North Korea's missile capabilities.

The US believes North Korea may have developed a missile with the capacity to target the US west coast.