Mattis told reporters Sunday he wants to "take measure" of China's strategic ambitions after it positioned weaponry on disputed islets in the South China Sea and is seeking to project its military power deep into the Pacific.

But in a four-day trip that will also include South Korea and Japan, the Pentagon chief also hopes to confirm China's commitment to pressuring North Korea to give up its nuclear weapons, after the historic talks between President Donald Trump and Kim Jong Un in Singapore in early June.

The United States, China, Japan and South Korea "have a common goal: the complete, irreversible and verifiable denuclearisation of the Korean peninsula," Mattis said.

In Beijing From Tuesday to Thursday, Mattis will meet with senior Chinese defense officials.

Then he will travel to Seoul for talks with his South Korean counterpart Song Young-moo, followed by a stop Friday in Japan to see defense chief Itsunori Onodera.

Those meetings are aimed at reassuring both allies that Washington's regional defense commitment remains unchanged after Trump unexpectedly announced on June 12 that the US would suspend a major joint military exercise in South Korea following his meeting with Kim.