Beijing claims sovereignty over almost all of the South China Sea and has waved away competing assertions from other regional countries as well as an international ruling that its position has no legal basis.

China and the Philippines have engaged in months of confrontations in the contested waters, and Manila is currently taking part in sweeping joint military drills with the United States that Beijing has slammed as destabilising.

The Chinese coast guard "implemented maritime control" over the Tiexian Reef, also known as Sandy Cay, during the middle of April, state broadcaster CCTV reported on Saturday.

The tiny sandbank, part of the Spratly Islands, lies near Thitu Island, also called Pag-asa and the site of a Philippine military facility.

CCTV said the coast guard landed on Sandy Cay to "exercise sovereignty and jurisdiction" over the reef, carry out an "inspection" and "collect video evidence regarding the illegal activities of the Philippine side".

The broadcaster published a photograph of five black-clad people standing on the uninhabited reef as a dark inflatable boat bobbed in the nearby water.

Another shot showed four coast guard officials posing with a national flag on the reef's white surface, in what CCTV described as a "vow of sovereignty".

The group also "cleaned up leftover plastic bottles, wooden sticks and other debris and garbage on the reef", the broadcaster said.

The Financial Times reported an unnamed Philippine maritime official as saying that the Chinese coast guard had left after unfurling the flag.

There do not appear to be any signs that China has permanently occupied the reef or has built a structure on it.

In recent months, Beijing and Manila have blamed each other for causing what they describe as the ecological degradation of several disputed landforms in the South China Sea.

China's state news agency Xinhua on Friday cited a report from the natural resources ministry that it said "debunked" Manila's allegations that Beijing's land reclamation projects had harmed the local environment.

Philippine forces are present on Thitu Island and Manila inaugurated a coast guard monitoring base there in 2023 in an effort to counter what it describes as Chinese aggression.

On Monday, the Philippine and US militaries launched three weeks of annual joint exercises called "Balikatan", or "shoulder to shoulder", which will include an integrated air and missile defence simulation for the first time.

US Marine Corps Lieutenant General James Glynn said at the opening ceremony in Manila that the two side would "demonstrate not just our will to uphold our mutual defence treaty in existence since 1951 but our matchless capability to do so".

"Nothing builds bonds more quickly than shared adversity," he said, without specifying a common threat.

Beijing said the manoeuvres "undermine regional strategic stability" and accused Manila of "collusion with countries outside the region".