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 U.S. 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 on Friday evening, said there was concern in the region that the U.S. 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.

U.S. 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 neighbors 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.

Later, answering questions, Turnbull said China could play a key role in reining in North Korea's nuclear and missile programs.

"China has overwhelmingly the greatest leverage over (North Korea). With that leverage comes the greatest responsibility," he said. "The eyes of the world are on Beijing."

Mattis told reporters he would talk about the "international order" needed for a peaceful Asia, a reference to countering North Korea, when he addressed the forum on Saturday.

"At the Shangri-la Dialogue I will emphasize the United States stands with our Asia-Pacific allies and partners," Mattis told reporters.

"The Department of Defense is focused on strengthening alliances, empowering countries to be able to sustain their own security, and strengthening U.S. military capabilities to deter war," Mattis said.

In the latest sign of increased pressure on North Korea, Japan's navy and air force began a three-day military exercise with two U.S. aircraft carriers in the Sea of Japan on Thursday.

David Helvey, a senior U.S. defense official dealing with Asian and Pacific security affairs, said the move was about sending a message of reassurance and "a message of resolve." He added that while these operations were routine, this was the first time operations with two U.S. carrier strike groups had taken place in the Sea of Japan since the late 1990s.

Additionally, the United Nations Security Council will vote on Friday on a U.S. and Chinese proposal to blacklist more North Korean individuals and entities after the country's repeated ballistic missile launches.

Mattis is expected to meet his counterparts from a number of countries including South Korea, Japan and Australia. China's delegation is led by a retired major-general from the Academy of Military Science, according to the forum's program.

The 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. Pyongyang has vowed to develop a nuclear-tipped intercontinental ballistic missile (ICBM) capable of hitting the U.S. mainland.

Some Asian officials say worries about Trump's direction have been fueled by his unpredictable personal approach to policymaking and emphasis on his chemistry with Chinese President Xi Jinping.

Since meeting Xi in April, Trump has praised him for efforts to restrain North Korea.

U.S. officials, however, insist the administration remains committed more broadly to the region, much like it was under former President Barack Obama's administration.

Trump is due to attend regional summits in Vietnam and the Philippines in November.

Mattis said he would talk about the need for countries to uphold international law, an apparent reference to Beijing's construction activities on disputed islets and reefs in the South China Sea.

China's claims to most of the South China Sea, through which about $5 trillion in ship-borne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

Last week, a U.S. Navy warship sailed within 12 nautical miles of an artificial island China has built on a disputed reef in the South China Sea, the first such challenge to Beijing in the strategic waterway since Trump took office.