South Korea's top nuclear envoy left for China Thursday for talks on North Korea, shadowed by signs that Pyongyang is preparing an imminent long-range missile test.

Lim Sung-Nam, Seoul's chief negotiator to six-party talks on North Korea's nuclear programme, was to meet with his Chinese counterpart Wu Dawei and other Beijing officials during his two-day trip.

Prior to departure, Lim told the Yonhap news agency that his meeting would focus on an "exchange (of) views on the recent situation on the Korean Peninsula".

Satellite operator DigitalGlobe Inc. recently released new images showing increased activity at North Korea's Sohae (West Sea) Satellite Launch Station that suggested a possible missile test in the next three weeks.

DigitalGlobe said the type of activity was consistent with preparations observed before North Korea's failed launch of its Unha-3 missile in April.

South Korean military officials have also cited intelligence reports pointing to a test sometime in December or January.

Pyongyang insisted the April launch bid was aimed at putting a satellite in orbit, but the United States and United Nations denounced the mission as a disguised ballistic missile test.

North Korea is known to have an inter-continental ballistic missile (ICBM) in development -- the Taepodong-2 -- but it has never been tested successfully.

Days after the failed April test, North Korea raised eyebrows by displaying what appeared to be a new set of ICBMs at a military parade to mark the 100th birthday of the North's late founder Kim Il-Sung.

But Western military analysts and UN sanctions experts concluded that the display models were simply mock-ups.

The six-party talks, triggered by Pyongyang's decision to withdraw from the nuclear non-proliferation treaty in 2003, comprise representatives from North and South Korea, China, the United States, Russia and Japan

Pyongyang walked out of the negotiations in April 2009, a month before it carried out its second atomic weapons test.