North Korea seems to be preparing to test-fire its longest-range missile, a US official and reports said Tuesday, a move that would heighten tensions amid stalled disarmament talks and icy relations with South Korea.

"There are some signs that the North Koreans are preparing for a Taepodong-2 launch," the US counter-proliferation official said.

"But whether it will carry out the launch or not is entirely unclear, as is the timing for a possible launch," said the official, who spoke to AFP on condition of anonymity.

Earlier, a source quoted by Seoul's Yonhap news agency said US and South Korean intelligence agencies had recently spotted a train carrying a long cylindrical object believed to be a Taepodong-2 missile which is theoretically capable of hitting the United States.

Launch preparations were likely to be completed in a month or two at a new west coast site, the source said. South Korea's defence ministry and National Intelligence Service refused to comment.

Japan's Sankei Shimbun newspaper said spy satellites had detected a large container capable of housing a missile being delivered to the site at Tongchang, about 40 kilometres (25 miles) south of the border with China.

The paper said frequent truck movements had been spotted at the site and launch preparations could be completed in one or two months for what could be a remodelled version of the Taepodong-2.

The missile has a maximum range of 6,700 kilometres (4,150 miles), meaning it could theoretically target Alaska.

Analysts said the North was trying to push the new US administration back to the negotiating table and to strengthen its bargaining position.

Nuclear disarmament talks with the North, involving the US and four regional powers, are deadlocked over how the communist state's atomic disclosures should be verified.

Reports of the planned launch also come amid rising tensions with Seoul. The North announced Friday it was cancelling all peace accords with its neighbour and on Sunday warned of a possible military conflict.

Ryoo Kihl-Jae, of the University of North Korean Studies, told Yonhap Pyongyang was angling for quicker dialogue with Washington amid frayed ties with Seoul.

Baek Seung-Joo of the Korea Institute for Defense Analyses said there would be a significant time gap between preparations and any launch.

"North Koreans seek warmer ties with the Obama administration, not strained relations, at the beginning," he told AFP.

Baek said the North wanted to get the attention of the US administration and gain the upper hand in upcoming negotiations with it.

North Korea staged an atomic test in 2006 and is thought to have enough plutonium for six or so bombs. Experts differ on whether it has miniaturised a bomb that could fit on a missile.

"I don't think the North Koreans are yet capable of producing a sophisticated nuclear warhead device to fit on a long-range missile," Baek said.

North Korea sparked regional alarm in 1998 by launching a shorter-range Taepodong-1 missile over Japan from its east coast launch site at Musudan-ri.

In July 2006 it launched a Taepodong-2 missile from there but US officials said it failed after about 40 seconds.

Daniel Pinkston, senior analyst with the International Crisis Group think-tank, said the launch site being built at Tongchang could be completed by the end of this year and a launch could be held even before then.

He said any long-range missile launch "would get the attention of the Obama administration. There are so many things on Obama's plate, but this would force everyone to take notice."

Pinkston said any launch might put a satellite in orbit.

Whatever the motive, such an achievement would earn leader Kim Jong-Il "tremendous domestic political benefit" -- especially if it preceded South Korea's first domestic satellite launch planned for June.

earlier related report
US, SKorea vow to end NKorea nuclear weapons: WHouse
US President Barack Obama spoke with South Korean President Lee Myung-Bak on Monday and the two vowed to work closely toward ending North Korea's nuclear weapons programs, the White House said.

The two "discussed North Korea and agreed to work closely as allies and through the six-party talks to achieve the verifiable elimination of North Korea's nuclear weapons and programs," spokesman Robert Gibbs said in a statement.

"In a warm and substantive talk, the president conveyed his deep commitment to the United States-Republic of Korea alliance. Both presidents expressed their intention to expand cooperation on global issues," Gibbs added.

The pair also "discussed the current financial crisis and agreed to work together, including at the G-20 Summit in London, to stabilize the global economy, to spur growth, and to get credit markets flowing."

In Seoul, presidential spokesman Lee Dong-Kwan said the accord came during a phone conversation that lasted about 15 minutes.

"Regarding the North Korean nuclear issue, President Obama said it is important for the countries to increase their cooperation in the six-party talks," the spokesman told a briefing.

"President Lee expressed thanks to the new US administration for showing a resolute attitude regarding the North Korean nuclear issue."

The US leader also said "a series of recent developments" had led him to realize that the denuclearisation of the Korean Peninsula can be achieved at an early date through cooperation among six nations, the spokesman said.

The two leaders said they expect to have "in-depth discussions" when US Secretary of State Hillary Clinton visits South Korea this month during her Asian trip, which also covers China and Japan, the spokesman said.

No date has been fixed yet, but news reports said Clinton is expected to make the trip around mid-February.

The six nation talks -- which group the two Koreas, the United States, China, Japan and Russia -- began in 2003 to disarm nuclear-armed North Korea.

A six-nation deal signed in February 2007 offers the North energy aid, normalized ties with Washington and Tokyo and a permanent peace pact if it dismantles its atomic plants and hands over all nuclear weapons and material.

But the disarmament talks are stalled by disagreements over how the North's declared nuclear activities should be verified.

Earlier Monday, North Korea's military vowed to keep atomic weapons until the United States removes its nuclear threat, reiterating its tough stance.

"The DPRK (North Korea) will never 'dismantle its nuclear weapons' unless nukes in South Korea are dismantled to remove the nuclear threat from the US," a spokesman for the North's General Chiefs of Staff was quoted as saying by the official Korea Central News Agency.

South Korea denies having any atomic weapons.