By Idrees Ali and David Brunnstrom
WASHINGTON, May 31 (Reuters) - U.S. Defense Secretary Jim Mattis is expected to press for greater cooperation to contain North Korea's nuclear and missile threat at a regional security forum in Singapore later this week, where for years Washington has sought to spotlight China's expansion in the strategic South China Sea.
President Donald Trump has actively courted Chinese support on North Korea and abandoned an Asia-Pacific trade pact soon after taking office in January. That has raised concerns among Southeast Asian allies in the lead up to the annual Shangri-la dialogue starting on Friday that Washington might allow China a freer rein elsewhere in the region.
A U.S. Navy challenge to China's South China Sea claims last week may have allayed some of the concerns over broader American engagement. But Asian officials and diplomats are still hoping for a clearer articulation of Trump's policy when Mattis speaks in Singapore on Saturday.
Abraham Denmark, a former deputy assistant secretary of defense for East Asia, said that while Mattis' focus would have to be on North Korea, perhaps Trump's most pressing security worry, it was important for him to lay out a broader policy to ease worries.
"The region will pay very close attention ... and the goal for Secretary Mattis should be to clearly enunciate the policy of the United States towards the Asia-Pacific," he said.
U.S. focus on North Korea has been sharpened by dozens of North Korean missile launches - the most recent of which was on Monday - and two nuclear bomb tests since the beginning of last year and by Pyongyang's vow to develop a nuclear-tipped intercontinental ballistic missile (ICBM) capable of hitting the U.S. mainland.
U.S. officials, speaking on condition of anonymity, said that in Singapore Mattis would likely stress the threat posed by North Korea and the importance of Asia-Pacific countries working together to counter its weapons programs as the United States negotiates with China and others to toughen sanctions on Pyongyang.
While Mattis' speech was still being worked on and could change, concerns about North Korea, one official said, would inevitably mean less emphasis on China's fortifying of artificial islands in the South China Sea. The building of islands is seen as a move by China to solidify its contested claims in the sea, one of the world's busiest shipping lanes.
U.S. officials and diplomats say that in spite of Trump's overtures to Beijing, China still appears reluctant to agree to tougher sanctions unless North Korea escalates tensions further by conducting another nuclear test or a launch directly related to its ICBM program.