Gary Locke on Monday formally resigned as US commerce secretary and became the new US ambassador China, promising to reap benefits from a difficult and sensitive US-China relationship.

Sworn in by Secretary of State Hillary Clinton who alluded to the challenges he will face, Locke, the first Chinese American to hold the prestigious post, said both the United States and China had much to gain from each other.

The two "have a profoundly important and complex diplomatic, economic and strategic bilateral relationship, one with challenges, no question, but one which also holds great promise for extended cooperation and collaboration," Locke told a State Department ceremony.

He said he would continue cooperative work to curb climate change, seek cleaner sources of energy and stop nuclear proliferation as well as efforts he pursued as commerce secretary to open Chinese markets to US goods and services.

The new envoy succeeds Republican former Utah governor Jon Huntsman, who left Beijing some three months ago and has mounted a run against former boss President Barack Obama in the November 2012 elections.

Locke also vowed to manage areas where he said the two countries have "differences" or "serious disagreement."

Locke, 61, takes over at a time of tensions in US-China relations, which have been fraught over a range of disputes including Beijing's export-driven economic policies, growing military spending and human rights record.

The two countries also have differences over US arms sales to Taiwan, US support for Tibet and China's pursuit of claims over potentially resource-rich islands in the South China Sea.

"I will work to keep the lines of communication open, to convey the administration's positions clearly and to engage with Chinese government officials at the highest levels," Locke said, referring to "sensitive" areas.

He also hoped "to do more to communicate directly with the Chinese people to improve understanding between our two great nations."

In May, Obama nominated John Bryson to serve as the next commerce secretary. Pending his confirmation, acting deputy commerce secretary Rebecca Blank will serve as acting commerce secretary.

In a Commerce Department statement announcing his resignation as commerce secretary, Locke touted what he said were increased exports, a reduced backlog for patent applications and a first step toward reforming export controls.