SAN DIEGO • US Defence Secretary Ashton Carter has said America's alliance with the Philippines remains "ironclad" even though the Asian ally's leader has vowed to end joint military exercises.

The Pentagon chief was speaking on Thursday as he headed for a security summit in Hawaii, where concerns about Philippine President Rodrigo Duterte, China's military expansion in the South China Sea, and the return of Islamic State in Iraq and Syria (ISIS) militants to the region are high on the agenda.

"As it has been for decades, our alliance with the Philippines is ironclad," Mr Carter said, addressing troops aboard the USS Carl Vinson aircraft carrier, docked in San Diego.

Mr Duterte on Wednesday said he would soon end joint military exercises with the United States, a symbolic blow to a military alliance dating back more than 60 years.

"I will serve notice to you now, that this will be the last military exercise, jointly Philippine-US, the last one," Mr Duterte told several hundred Vietnam-based Filipinos during a rambling address in Hanoi at the start of his Vietnam trip.

But senior defence officials later appeared philosophical about Mr Duterte's outbursts. The US-Philippine alliance has "had its ups and downs and survived", one official said, speaking on condition of anonymity. "It's going to continue to survive based on what we think are strong US-Philippine common security interests."

As for the military exercises, "we can be flexible about when and if they happen again", the official added.

Mr Carter was set to meet his Philippine counterpart Delfin Lorenzana in Hawaii yesterday, where the Pentagon chief is hosting an "informal" meeting for Asean defence ministers.

Previous summits have tended to focus on China and its growing reach across the South China Sea. Beijing has in recent years rapidly expanded its physical presence in the strategically vital waterway, turning small maritime features, islets and reefs into much larger islands capable of holding military facilities.

Mr Carter stressed, as he has repeatedly, that the US military will ignore Beijing's contentious South China Sea claims, and keep operating in waters and airspace surrounding the islands.

"We hope that China chooses to join the rest of the region in strengthening and upholding the shared principles that have helped so many nations around the region, including China, to rise and prosper," he said.

Several nations in the region, including Indonesia, Malaysia and Singapore, are also concerned about the return of Islamists from the Middle East as ISIS loses territory in Iraq and Syria.

The defence official said probably about 1,000 or fewer South-east Asians are in Syria and Iraq, and "hundreds" have already returned to their home countries.

"Local governments certainly are very concerned about the possibility of a rise in ISIL-related violence. It is very high on their agenda when they speak to us," the official said, using another acronym for ISIS.

Regional allies are sure to ask Mr Carter about the upcoming US election, which has seen a tightening race between Mr Donald Trump and Mrs Hillary Clinton.

The defence official indicated he was confident the US would retain its Asia focus whatever the election's result.

"We've got a good thing going with the establishment of this principled, inclusive security network," he said. "There are a lot of opportunities left over. And we want to make sure that our position in the region is solid for the next president."

AGENCE FRANCE-PRESSE