"We really don't know what are the intentions of chairman Kim Jong-Il or any other military leaders in North Korea," Vershbow said in an interview with state-run KBS radio.

"But as I said we want them to understand that tests in light of other problems that we have in our relations with North Korea will be viewed as a very serious matter and we will take appropriate measures in response."

He said consultations would take place with partners to the six-party talks with North Korea before a decision would be made on a response.

On-off talks on ending the North Korea nuclear standoff have brought together the two Koreas, China, Japan, Russia and the United States since August 2003.

South Korea's foreign minister urged Pyongyang to abandon plans for the missile test that would have a "negative impact on the international geopolitical situation and the settlement of North Korea's nuclear issue."

"It is appropriate for North Korea to stop preparations for the missile launch and return to six-party talks," Ban Ki-Moon told a weekly media briefing.

A senior South Korean official who declined to be named earlier confirmed reports that North Korea was preparing to test-fire a Taepodong-2 multi-stage missile with a range of more than 6,000 kilometres (3,600 miles) that was capable of hitting the United States.

"Preparations which were reported last month have not stopped," the official said.

Separately a US official was quoted as saying there were "enough indications" to suggest that the Stalinist state was preparing to test-fire the missile.

Ban declined to say how far North Korea's launch preparations had progressed and gave no estimate of when a launch could be ready, but he said South Korea was closely watching developments.

Current test preparations are reportedly far more advanced than on previous occasions when North Korea appeared to be gearing up for a missile launch.

In 1998 Pyongyang shocked the world by launching a Taepodong-1 missile that flew over Japan before crashing into the Pacific.

The launch triggered jitters in Japan and prompted Tokyo and Washington to accelerate plans to build a missile defense system.

Vershbow said North Korea should abide by a long-range missile moratorium declared by its leader Kim Jong-Il in 1999.

"We really call upon North Korea to abide by the moratorium on flight tests of long-range missiles," he said.

"If they were to test a long-range missile now it would be a very serious matter and we will consider it to be very much against the spirit of the September agreement of the six-party talks."

North Korea agreed at six-party talks in September to dismantle its nuclear weapons programme in return for economic aid and other benefits. But in November Pyongyang said it would boycott further talks after Washington imposed financial sanctions for alleged counterfeiting and money laundering.

Some experts say North Korea, aware that launch preparations are easily monitored by US satellite, wants to use the threat of a test-flight to force Washington to roll back the sanctions.

Reports of a possible launch surfaced last month in Japanese and South Korean newspapers. They said US satellite images showed 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).

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