US committed to Asia-Pacific allies: Mattis

US Defense Secretary Jim Mattis said yesterday the United States remains committed to its Asia-Pacific allies, as he arrived in Singapore for the region's premier security forum.

Mattis, who is making his second visit to the region since he took charge of the Pentagon on Jan. 20, will be looking to articulate a clear US policy for allies in the region and reassuring them at the annual Shangri-La dialogue in Singapore.

Australian Prime Minister Malcolm Turnbull, who spoke at the opening of the forum yesterday evening, said there was concern in the region that the US withdrawal from the Trans-Pacific Partnership and the Paris climate accord would lead to Washington retreating from global leadership.

"While these decisions are disappointing, we should take care not to rush to interpret an intent to engage on different terms as one not to engage at all," Turnbull said.

Authorities in Singapore stepped up security as Turnbull, Mattis and other leaders arrived for the meeting. They have said the terrorism threat to the city-state remains at the highest level in years, although there was no credible intelligence of an imminent attack.

However, last week's dramatic attack in the Philippines by militant groups owing allegiance to Islamic State has created jitters in the region.

Armed troops patrolled the area around the venue for the Singapore forum and nearby mailboxes were sealed.

US President Donald Trump has actively courted Beijing's support on North Korea, raising concerns among Southeast Asian allies in the lead-up to the dialogue that Washington might allow China a more free rein elsewhere in the region.

Turnbull said the region wanted to see China take a responsible leadership role or risk becoming isolated.

"A coercive China would find its neighbours resenting demands they cede their autonomy and strategic space, and look to counterweight Beijing's power by bolstering alliances and partnerships, between themselves and especially with the United States," he said.