"What seems sure is that they have assembled this missile on the launching pad," he said in Geneva where he is attending the opening session of the UN's Human Rights Council.

A top South Korean intelligence official said North Korea appeared to be still fueling the missile, contrary to earlier reports.

The official confirmed at a briefing that preparations for a launch were under way at Musudanri in the North's remote northeastern province of North Hamgyong.

According to a senior US official quoted by the New York Times Monday, satellite images suggested that fuelling had been completed. Booster rockets had been loaded onto a launch pad and liquid fuel tanks fitted to the missile, the newspaper said.

But the unidentified South Korean intelligence official said North Korean technicians appeared to be still filling the "launching vehicle", which required some 60 tonnes of fuel for a test.

"It is difficult to determine that fueling has been completed, judging from the number and size of the fuel cans that are seen on the site," the National Intelligence Service (NIS) official told lawmakers at a secret intelligence briefing, according to lawmaker Chung Hyung-Keun.

Preparations for the launch have triggered jitters in Asia and drawn sharp warnings from Washington and Tokyo, while South Korea has repeatedly urged its neighbour to call off the test.

The Japanese foreign ministry said late Tuesday that Japan and South Korea had agreed to cooperate to prevent North Korea from launching the ballistic missile.

The two nations reached the agreement during a 25-minute telephone conversation between the Japanese and South Korean foreign ministers, Japan's Kyodo news agency reported.

North Korea was believed to be readying the launch of a Taepodong-2 missile that can carry a 1,000-kilogram warhead up to 6,700 kilometres (4,200 miles), far enough to hit targets in Alaska and possibly Hawaii.

A North Korean researcher said late Tuesday any such launch was a sovereign "right" although he did not confirm preparations were under way.

A launch concerns "the right of autonomy of a country, which no one has the right to defame," Ri Pyong-Dok, a deputy chief-level researcher at the foreign ministry, told Japanese reporters visiting Pyongyang.

The United States and Japan have warned North Korea of serious consequences if the launch takes place.

The North would face deeper isolation, said US Ambassador to South Korea Alexander Vershbow on Tuesday after meeting with former South Korean president Kim Dae-Jung. "We both agreed carrying out the test at this time would further compound North Korea's isolation and put it more apart from the international community," Vershbow said after the meeting with Kim, who plans to visit Pyongyang next week.

UN Secretary General Kofi Annan added his concern, warning Pyongyang not to further escalate tensions in the region.

"I hope North Korea's leaders will listen and hear what the world is saying. We are all highly concerned, they must be very careful to avoid creating an even more complicated situation on the (Korean) peninsula," Annan told reporters in Paris.

President George W. Bush and other top administration officials have been in contact with leaders around the world to discuss North Korea, White House spokesman Tony Snow said.

At the United Nations, US ambassador John Bolton said he was consulting members of the Security Council to decide what steps might be taken if North Korea carried out the "very serious" act of a missile test.

Deputy Secretary of State Robert Zoellick said the United States was consulting partners in the six-nation talks on North Korea's nuclear weapons program, and other countries.

Analysts in Seoul said if the long-range missile was tested, North Korea would describe it as a satellite launch as it did in 1998 when it fired a missile over Japan.

North Korea caused alarm in August 1998 by firing a Taepodong-1 missile with a range of up to 2,000 kilometers over Japan into the Pacific Ocean. It later claimed it had succeeded in launching a satellite into orbit.

The North declared a moratorium on flight tests of long-range missiles in 1999 but said in 2005 that it would no longer observe it.

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