Seoul: China has expressed "serious concern" to its ally North Korea about a planned rocket launch, a US official said on Tuesday after talks between President Barack Obama and President Hu Jintao.

The leaders met a day after Obama made an unusually blunt appeal to Beijing to get tougher with Pyongyang.

Numerous nations have strongly criticised the launch set for mid-April. The nuclear-armed North insists it will only put a satellite into orbit, while its opponents say it will test missile technology that could deliver a warhead.

China, the North's sole major ally and its biggest trade partner and aid provider, is seen as one of the few nations that can influence the regime.

"The two leaders agreed to coordinate closely in responding to this potential provocation and registering our serious concern to the North Koreans and, if necessary, consider what steps need to be taken following a potential satellite launch," said Ben Rhodes, a deputy national security adviser.

Given China's influence, Obama "felt it was very important for us to be working closely with China and for China to be sending a very strong message to North Korea", Rhodes said.

The Chinese "have indicated to us that they take this very seriously, that they've registered their concerns with the North Koreans," he told reporters.

Beijing would work actively with Washington and other six-party talks members "to make clear to the North Koreans the very grave concerns that the international community has if they go forward with this provocative act".

The now-stalled six-party forum, grouping the two Koreas, Russia, China, the US and Japan, has been trying since 2003 to negotiate an end to the North's atomic weapons programme.

Obama and Hu met before the start of a nuclear terrorism summit in South Korea which has been overshadowed by the launch and by Iran's suspected attempts to develop nuclear weapons.