The communist regime, which is boycotting nuclear disarmament talks, could fire for the first time a 35-meter (116-foot) Taepodong-2 in the range of 3,500 to 6,000 kilometers (2,200 to 3,750 miles), officials said.

North Korea is believed to be developing the missile for a range of up to 10,000 kilometers, which would put the continental United States within striking distance.

Foreign Minister Taro Aso, however, doubted the launch would come immediately.

"We have known that information for a long time," Aso told a legislative committee.

He was responding to media reports from Tokyo and Seoul that satellite data have shown increased movement by trailers and other vehicles near the Musudan-ri missile test site in northeastern North Korea, facing the Sea of Japan (East Sea).

Aso declined to comment on when Tokyo acquired the information, citing the sensitivity of intelligence matters.

Other senior officials also played down the risks.

"At this point, we do not regard the situation as critical," said Fukushiro Nukaga, head of the Defense Agency.

Chief Cabinet Secretary Shinzo Abe, the government spokesman, said separately: "We do not hold a view that this is an urgent situation."

News reports said Japanese and South Korean officials learned about the increased activity from US forces based in the two countries.

A military source told Yonhap news agency that South Korean and US military authorities detected some signs of activity earlier this week in the North.

"But they are still analyzing whether North Korea will move to launch a missile or if it is merely a demonstration against US pressure to give up its nuclear weapons programs," the source was quoted as saying.

South Korean foreign ministry spokesman Bae Young-Han said the reports were not confirmed.

"The government has already been closely watching (North Korea)," he told reporters.

North Korea shocked the world in August 1998 by firing a long-range Taepodong-1 missile with a range of up to 2,000 kilometers over Japan into the Pacific Ocean, claiming it was a satellite launch.

It has since carried out a series of tests on smaller-range missiles.

Tokyo has been in a hurry to bolster its missile defenses and in March hailed the successful testing of an interceptor missile being researched with the United States.

North Korea, which says it has nuclear arms, has refused to return to six-nation disarmament talks since November, protesting US financial sanctions against the impoverished regime over money-laundering and counterfeiting.

Washington has also denounced Pyongyang as a leading global proliferator of missiles and missile technology. The cash-strapped communist state has refused to stop missile exports, a major source of hard currency earnings.

In June last year, North Korean leader Kim Jong-Il told a South Korean envoy he would scrap the missiles once diplomatic ties were established with Washington.

North Korea has short-range Scud missiles targeting South Korea and intermediate-range Rodong missiles that can hit targets up to 1,300 kilometres (812 miles) away, including most parts of Japan.