Recent satellite images show that North Korea has completed a launch tower at its new missile base, a key step in efforts to test a missile which could eventually reach the United States, experts say.

An image taken on January 10 of the Tongchang-ri base on the west coast shows a moveable launch pad and swing arms along with the tower.

It was disclosed by VOA News this week and was posted on the website of US defence information group GlobalSecurity.org, prompting a senior US military commander to describe the development as a "major concern".

The new base is seen as a key step in the North's quest for an intercontinental ballistic missile (ICBM) that could possibly strike the United States, GlobalSecurity.org said.

It is bigger and more advanced than the Musudan-ri base on the east coast, which the North used to launch long-range missiles in 1998, 2006 and 2009.

The North has enough nuclear material for an estimated six to eight weapons but it is unclear whether it has the technology to create a nuclear warhead for a missile.

US Defence Secretary Robert Gates warned last month that North Korea could have missiles within five years that would directly threaten the United States.

"This is a major concern of ours," Admiral Robert Willard, head of the 300,000-troop US Pacific Command, said of North Korea's missile programme.

"When you package that together with the provocative actions that we saw in 2010, and the complexities of succession that are currently ongoing in North Korea, it should concern us all," Willard said at the Asia Society.

But Willard played down speculation of an imminent missile launch by North Korea. There are "no signs that I'm aware of that they're preparing for near-term missile tests," Willard said.

Work at Tongchang-ri has been monitored for more than two years and South Korean officials said in October 2009 that construction was near completion. But the latest images were the first to show a launch tower.

Daniel Pinkston, a Seoul-based analyst with the International Crisis Group, said Tongchang-ri clearly had more facilities to support a missile development programme than the relatively "primitive" Musudan-ri.

"It demonstrates their commitment to an ICBM programme," he told AFP, "considering the cost of the programme, the small size of their economy and their technical capabilities.

"If they are going to dedicate such resources, it's a sign they are serious about using the launch base," Pinkston said, adding that a test-launch is possible this year.

The North's first long-range test in 1998 sent a Taepodong-1 missile over Japan but failed to put a satellite in orbit. A Taepodong-2 exploded after 40 seconds after launch in 2006.

In April 2009, another Taepodong-2 travelled some 3,200 km (1,984 miles) to land in the Pacific.

That launch, and a nuclear test a month later, brought fresh UN sanctions including a ban on missile and nuclear-related activity.

Inter-Korean relations are icy after two deadly border incidents last year blamed on Pyongyang. Six-party nuclear disarmament talks have been stalled since December 2008 and Washington is resisting appeals for direct dialogue.

A long-range missile programme could be used as a bargaining chip to extract US concessions.

Pinkston said a successful launch would also have huge domestic propaganda value and boost the prestige of Kim Jong-Un, youngest son and heir apparent to leader Kim Jong-Il.

Any nuclear strike capability by the North could also be perceived as undermining the US commitment to come to the aid of its regional allies South Korea and Japan, he added.