South Korea's top nuclear envoy left for Washington Tuesday for talks on North Korea's planned rocket launch, as Seoul urged China to play an active role in getting Pyongyang to cancel the mission.

Lim Sung-Nam, Seoul's chief delegate to the stalled six-party talks on the North's nuclear programme, will meet with senior US State Department and National Security Council officials during his three-day visit.

Before his departure, Lim said Seoul and Washington would "maximise diplomatic efforts and the coordination between South Korea and the US to block North Korea's launch".

Pyongyang's announced on Saturday that it intends to launch a long-range rocket between December 10 and 22.

The United States and its key Asian allies South Korea and Japan have condemned the move as a disguised ballistic missile test that violates UN resolutions triggered by Pyongyang's two nuclear tests in 2006 and 2009.

In a briefing in Seoul, foreign ministry spokesman Cho Tai-Young again called on the North to call off the launch and abide by its obligations under the relevant UN resolutions.

"If the North pushes ahead with the launch, it will face consequences," he said, without elaborating.

Cho said China, North Korea's sole major ally and its biggest trade partner and aid provider, had a "crucial" part to play.

"We hope China will play an active role in keeping peace and stability on the Korean peninsula", he said.

Envoy Lim on Monday met ambassadors in Seoul from China, Russia and Japan -- other members of the six-party talks on North Korea -- to discuss a common response to the planned launch.

Pyongyang insists the launch is a "peaceful" and purely scientific mission aimed at placing a satellite in orbit.

A previous attempt in April failed when the carrier exploded shortly after take-off.

China has expressed "concern" at the launch plan, with the foreign ministry urging "relevant parties (to) act in a way that is more conducive to the stability of the Korean peninsula".

Russia added its "regret" at Pyongyang's announcement and noted that North Korea was obliged to abide by UN resolutions.

UN chief Ban Ki-moon is "seriously concerned" that the launch would heighten tensions in the region and urged the North to re-consider its decision, his office said in a statement Tuesday.

"He further calls upon (North Korea) to re-establish its moratorium on missile launches, as required by the Security Council," the statement said.

Analysts say the international community is running out of options for pressuring the impoverished but nuclear-armed North, which is already under layers of sanctions.

The six-party, aid-for-denuclearisation talks have been at a standstill since Pyongyang walked of the forum in April 2009. It staged its second nuclear test a month later.