MANILA • A United States warship laden with high-tech fighter planes took centre stage in key war games in the Philippines yesterday, brandishing military might as fresh tensions bubble in the contentious South China Sea.

The amphibious assault ship USS Wasp, with at least 10 F-35B stealth jets on deck, stood guard as amphibious tanks rolled onto a Philippine beach located a short sail away from islands also claimed by China.

The vessel was in the area for long-running US-Philippine military drills, which come as Manila pushes back against the recent presence of hundreds of Chinese ships near its Pag-asa (Thitu) island.

"We can't disclose the official movements of the USS Wasp for security reasons, but (it has) been operating in the South China Sea region... as part of the exercise," a US spokesman, Second-Lieutenant Tori Sharpe, told reporters.

President Rodrigo Duterte had largely set aside the once-tense dispute over China's expansive claims to the resource-rich waterway, but told Beijing last week to back off over the "swarming" Chinese boats.

The Philippines called the boats' presence "illegal" and Mr Duterte threatened China with possible military action if Beijing touches the island.

China claims most of the sea, including waters and islands close to the shores of its neighbours.

It has built artificial islands and military installations that the US warned could curtail the right of passage by non-Chinese vessels.

The Philippines, Brunei, Malaysia, Taiwan and Vietnam have also all staked claims to various islands and reefs in the sea that is thought to have rich petroleum reserves deep beneath its waters.

The USS Wasp's participation in the exercises represents "an increase in military capability committed to a free and open Indo-Pacific region", the US navy said in a press release on its arrival last week.

The two-week Balikatan (shoulder-to-shoulder) exercises, which opened on April 1, include some 4,000 Philippine, 3,500 American, and 50 Australian troops.

Mr Duterte had threatened to quit the drills as part of his pivot away from former colonial master the US and towards China, but the drills have carried on after being toned down for a few years. As Mr Duterte has adopted a friendlier stance with China, the US has moved to shore up its ties with Manila and assert its presence in the South China Sea.

US Secretary of State Mike Pompeo pledged last month to come to the Philippines' defence in case of an "armed attack" in the sea, a clear assurance Philippines leaders had long sought.

Mr Duterte has been criticised at home as being too eager to grow ties with Beijing, and giving up too much leverage on the South China Sea issue.

China has been reserved in its comments regarding Manila's recent uptick in assertiveness over the Pag-asa island, saying that it was working with the Philippines and looking into the reports regarding Chinese ships.

AGENCE FRANCE-PRESSE