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 Voice of America News this week and was posted on the website of US defence information group GlobalSecurity.org.
The new base is seen as a major 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.
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 today, "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.