US President Barack Obama has welcomed a bigger role for China on the world stage and pledged to pursue greater cooperation with Asian countries.

Speaking in Tokyo, he said the US would not be "cowed" by North Korea's nuclear threats and that his commitment to the security of Asia was "unshakeable".

Obama also called on Asian leaders to pursue balanced economic growth.

Early yesterday, he flew to Singapore for an Asia-Pacific economic summit a few hours earlier than planned.

His trade representative Ron Kirk, who is already at the Apec meeting, says the US wants barriers to trade and investment removed to promote an open global trade system.

Obama will round off his week-long Asian tour with stops in China and South Korea.

Calling himself America's first "Pacific president", Obama told the gathering in Tokyo that Washington's commitment to the region's security was "unshakeable" despite fighting wars in Iraq and Afghanistan.

He said the US did not seek to "contain" China's rising power, and pledged to pursue "pragmatic cooperation" with Beijing on issues of mutual concern.

"The rise of a strong, prosperous China can be a source of strength for the community of nations," Obama said.

He warned that he would not waver from raising human rights concerns with Beijing, but did not mention specific concerns, such as Tibet.

He sought more assistance from China to thwart the ambitions of North Korea, and warned there would be tough, unified action by the US and its Asian partners if Pyongyang failed to abandon its nuclear weapons programmes.

Obama again called on North Korea to return to six-party talks on the issue.

On the issue of economic cooperation, Obama challenged Asian countries to break their dependence on exports to the US and to pursue "balanced" and sustainable economic growth.

"We must strengthen our economic recovery, and pursue growth that is both balanced and sustained," he said. "We simply cannot return to the same cycles of boom and bust that led us into a global recession."