The top US military officer in the Pacific on Tuesday said Washington was "very, very concerned" about Beijing's recent "dangerous" actions in the disputed South China Sea, after confrontations with Philippine vessels.

Speaking in Sydney, United States Indo-Pacific Command head Admiral John Aquilino referenced recent encounters between Chinese and Philippine boats around a remote shoal in the Spratly Islands.

"I'm very concerned about what's happening at the Second Thomas Shoal", said Aquilino. "I'm very, very concerned about the direction it's going."

Since the 1990s, China has sought to exert control over a swathe of the southwest Pacific, bringing tense standoffs with many neighbours who claim various islands, atolls, reefs and shoals for their own.

The last month has seen two collisions near the Second Thomas Shoal between vessels from China and the Philippines, and Chinese ships blasting water cannon at Philippine boats.

Both countries claim rights over the area -- which sits at a vital resource-rich maritime crossroads for regional trade.

The United States, a treaty ally of the Philippines, has led a chorus of support for the Southeast Asian country in response to China's actions.

Aquilino said Tuesday that China's "unilateral" actions were "dangerous, illegal and they are destabilizing the region".

"What's next and how far are they willing to go in that area?"

China calls the Second Thomas Shoal, Ren'ai Reef, and claims it lies within the so-called "nine-dash line" -- a vast zone of Chinese control that encompasses almost all of the South China Sea.

In 2016, an international tribunal dismissed Chinese historic claims over the territory as having no legal foundation and criticised Beijing's naval actions as impinging on the Philippines' sovereignty.

But the ruling has been largely ignored by China and confrontations have continued.