The Chinese coastguard has seized a tiny sandbank in the South China Sea, state media has reported, in an escalation of a regional dispute with the Philippines.

State broadcaster CCTV released images of four officers, wearing all black and holding the Chinese flag, stood on the disputed reef of Sandy Cay in the Spratly Islands.

The channel said China had "implemented maritime control and exercised sovereign jurisdiction" on the reef earlier in April.

The Philippine government has yet to formally respond. Both China and the Philippines have staked their claims on various islands and zones. Their dispute has been escalating, with frequent confrontations including vessels colliding and scuffles.

Sandy Cay is near a Philippine military outpost on Thitu Island, which Manila reportedly uses to track Chinese movements in the area.

There is no sign that China is permanently occupying the 200 sq metre island and the coastguard is reported to have left.

The White House said reports of China seizing the reef were "deeply concerning if true".

In comments reported by the Financial Times, James Hewitt, US National Security Council spokesperson, warned that "actions like these threaten regional stability and violate international law", adding that the White House was "consulting closely with our own partners".

The Chinese move comes as US and Philippine forces are carrying out their annual war scenario drills - called the Balikatan exercises. China has criticised the drills as provocative.

As many as 17,000 personnel are taking part in the coming days. Missiles from the US Marine Air Defense Integrated System were fired off the coast of the northern Philippines on Sunday, the system's second live fire test and its first deployment to the Philippines. The drills are also set to feature the US anti-ship missile system NMESIS.

The Philippines military says the drills are a rehearsal for national defence but insists they are not directed at any particular country.

"This type of training is absolutely invaluable to us," said Third Marine Littoral Regiment Officer John Lehane.

The exercise has helped allay fears among some US allies that Donald Trump may upend the years-long military support it has provided in the region.

On a visit to Manila last month, US Defence Secretary Pete Hegseth said Washington was "doubling down" on its alliance with the country and was determined to "re-establish deterrence" against China.

There have been wrangles over territory in the South China Sea for centuries, but tension has grown in recent years.

China claims by far the largest portion of territory in an area demarcated by its so-called "nine-dash line". The line comprises nine dashes which extends hundreds of miles south and east from its most southerly province of Hainan. Beijing has backed its expansive claims with island-building and naval patrols.

Competing claimants such as Vietnam, the Philippines, Taiwan, Malaysia and Brunei have staked claims on islands and various zones in the sea.