MANILA – US national security adviser Robert O’Brien has assured the Philippines and Vietnam, countries both locked in maritime rows with China, that Washington has their backs and would fight to keep the Indo-Pacific region free and open.

“Our message is we’re going to be here, we’ve got your back, and we’re not leaving,” said O’Brien, on a visit to the Philippines after concluding a trip to Vietnam on Sunday.

“I think when we send that message – that peace-through-strength message – is the way to deter China. It is a way to ensure the peace,” O’Brien said.

Vietnam and the Philippines have been the most vocal regional opponents to what they see as Chinese overreach in the South China Sea and its disregard for boundaries outlined in international maritime law.

China claims 90% of the potentially energy-rich South China Sea, but Brunei, Malaysia, the Philippines, Taiwan and Vietnam each claim parts of it.

The United States has long opposed China’s expansive claims, sending warships regularly through the strategic waterway to demonstrate freedom of navigation there.

China maintains it is a force for peace in the region and sees the US presence as provocative and interference by an outsider.

O’Brien, who led the turnover in Manila of $US18 million worth of precision-guided munitions, said the United States stood with the Philippines in protecting its offshore resource entitlements.

“Those resources belong to the children and grandchildren of the people here,” he said.

“They don’t belong to some other country that just because they may be bigger than the Philippines,” he said, adding: “That’s just wrong.”

His visit came more than a week after the Philippines suspended its scrapping of a Visiting Forces Agreement (VFA) with the United States for a second time, as the treaty allies work on a long-term mutual defence arrangement.

Last year, US Secretary of State Mike Pompeo assured the Philippines it would come to its defense if attacked in the South China Sea.

Reuters