Filipino and Vietnamese troops will engage in a light-hearted round of sports diplomacy on contested islands in the South China Sea, the Philippines said Thursday, as tensions worsen with China.

Basketball, beach volleyball and tug-of-war games will be held this weekend on tiny islands that are claimed by nations to show that rivals can still be friends, Foreign Secretary Albert del Rosario said.

"This is camaraderie. That is a friendly gesture," del Rosario told reporters.

The games will be held in the Spratly archipelago, a powder keg area of the South China Sea that is coveted by the Philippines, Vietnam, China, Malaysia, Brunei and Taiwan.

All the claimants except for Brunei have for decades occupied islands in the Spratlys in a bid to block any of the others.

It is part of a broader struggle for control over the South China Sea, which has enormous trade and military significance as it is the main maritime link between the Pacific and Indian oceans.

China claims nearly all of the sea, which is also believed to contain vast deposits of oil and gas, even waters approaching the coasts of its neighbours.

The Philippines and Vietnam have voiced growing alarm in recent years at what they perceive as China's bullying tactics in staking its claims to the sea.

Vietnamese Prime Minister Nguyen Tan Dung said last month that China had become a serious threat to peace, after the deployment of a Chinese oil rig in the Paracel archipelago to the north of the Spratlys.

Philippine President Benigno Aquino expressed concern on Thursday at what he said were photographs showing more Chinese ships in the Spratlys that he feared could be used to reclaim land and convert tiny reefs into China-held islands.

"These were ships that can be used for (land) reclamation," Aquino told reporters.

He said the vessels were similar to ships China used recently to turn another reef in the Spratlys into a tiny island that the Philippines believes could eventually support an airstrip.

As the disputes with China have intensified, the Philippines and Vietnam have sought to work more closely together and share information about their common foe's actions in the sea.

This weekend's planned sports are intended to continue fostering a warmer relationship.

The games will be a tiny affair, with only a small number of soldiers taking part, according to Lieutenant General Roy Deveraturda, head of the Philippines' western military command that has responsibility for the Spratlys.

But he said they would see Filipino soldiers cross over to a Vietnamese-held island for day one of the games, with the Philippines to host Sunday's events.