Washington: A top US military officer has left for China in a trip designed to bolster a fledgling security dialogue with Beijing, even as a US naval exercise in the South China Sea threatens to upstage his visit.

Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, began the four-day tour that will include talks with senior officers and a visit to military units, officials said on Friday.

Mullen, who in May hosted his Chinese counterpart, People's Liberation Army Chief of General Staff Chen Bingde, "looks forward to continuing the engagement and dialogue" with Chen in Beijing, the Pentagon said in a statement.

But the Admiral's trip coincides with a joint naval exercise set for Saturday with the US, Japanese and Australian Navies in the South China Sea, where China has asserted territorial claims.

US and Japanese officials said the exercise will include the Japanese destroyer Shimakaze, an American destroyer -- the USS Preble -- and a Royal Australian Navy patrol boat.

The ships will carry out communications training and other drills off Brunei, officials said.

The US Navy played down the exercise, with a spokeswoman calling it a small-scale, "low-level" activity on the sidelines of an International Defence Exhibition in Brunei.

Lieutenant Commander Tamara Lawrence told a news agency it was a "passing exercise," which typically includes flag semaphore drills, navigation and other exercises focused on "basic seamanship."

China has objected to previous US naval drills in the South China Sea, and tensions in the strategic and resource-rich area have mounted in recent weeks.

The Philippines and Vietnam have expressed concern over what they call China's increasingly assertive stance in the area.

Mullen's visit also comes after the United States and the Philippines carried out joint naval exercises, which Manila and Washington insisted were aimed at deepening military ties and not related to worries over China.