SEOUL - A team of U.S. officials and nuclear experts crossed the heavily armed border into North Korea on Tuesday on a rare visit to survey the communist state's nuclear facilities.

They will be joined by experts from two other nuclear powers -- Russia and China -- at the invitation of Pyongyang in what Washington has called another key step towards finally ridding the Korean peninsula of atomic weapons.

Speaking to South Korean officials late on Monday, the head of the U.S. delegation, Sung Kim of the State Department, said the inspections "should set the stage for the next phase of disabling".

North Korea has agreed to fully account for and disable its nuclear weapons programme by the end of this year under a February deal with South Korea, the United States, Japan, Russia and China.

It has let in international nuclear inspectors and shut down its main Yongbyon nuclear complex, which had produced bomb-grade plutonium, in return for 50,000 tonnes of heavy fuel oil.

North Korea tested its first atomic device a year ago and is thought to have enough fissile material to make several nuclear warheads.

By completing full disarmament, the impoverished North will receive an additional 950,000 tonnes of oil or other aid of same value.

U.S. President George W. Bush has also offered a peace treaty with the North if it gave up its nuclear weapons programme.

The U.S. delegation crossed the border from the South through the Panmunjom truce village that straddles the Demilitarised Zone border drawn at the end of the 1950-53 Korean War.

It will be joined by Russian and Chinese officials in the North Korean capital Pyongyang and travel to Yongbyon, about 100 km (60 miles) north of the capital. Their visit is expected to end on Saturday.