China has systematically expanded its maritime militia in the South China Sea over the last decade as part of its strategy to assert more control over the contested waters, according to a new report by an American think-tank.

Since China completed construction of its artificial island outposts in 2016, more militia boats have been deployed more frequently to the Spratly Islands, highlighted the report by the Washington-based Centre for Strategic and International Studies, which was funded by the US State Department.

It claimed that the boats accompanied Chinese law enforcement in recent oil and gas stand-offs with Malaysia and Vietnam, and that about 200 vessels had gathered at Whitsun Reef in March, sparking a diplomatic incident with the Philippines.

The report, by the centre's Asia Maritime Transparency Initiative (AMTI), tracked the operations, home ports, funding and ownership of China's maritime militia in the South China Sea, in what it called the first comprehensive profile of the militia to date.

The report defined this force as primarily made up of "vessels ostensibly engaged in the business of commercial fishing, but whose true occupation is in achieving Chinese political and military objectives".

"What we discovered over the course of this research is that there's clearly been an effort to professionalise and build up the militia over the last eight years, coinciding with (Chinese President) Xi Jinping's elevation to power," said AMTI director and co-author Greg Poling at a webinar on Thursday when the report was released.

China did not respond yesterday to the report, but it has long denied that it uses maritime militia to press its claims in the disputed waters, and argues that the vessels are commercial boats fishing in those waters. Vietnam, the Philippines, Malaysia, Brunei and Taiwan also have competing claims in the area.

The AMTI report, however, cited several sources, including Chinese-language reports that explicitly referred to certain boats as maritime militia, satellite photos and ship tracking data that showed vessels in the disputed areas were not engaged in fishing.

"When vessels loiter for days or weeks without ever trawling or deploying gear, it is extremely compelling evidence they are not commercially fishing," said the report.

It said that another tell-tale sign was the vessels' practice of tying themselves together, or rafting up.

This increases their stability when anchored and enables them to communicate more easily among themselves during long periods of inactivity, said the report, which included a photograph of Chinese vessels tied together at Whitsun Reef in March.

"There is no commercial rationale for a large fleet of fishing vessels to operate in this manner. If they were full-time fishers, they would be losing money by the day," said the report.

Addressing the Whitsun Reef incident at the time, the Chinese Embassy in the Philippines had said that some Chinese fishing vessels took shelter near the area due to rough sea conditions.

"It has been a normal practice for Chinese fishing vessels to take shelter under such circumstances. There is no Chinese maritime militia as alleged," the embassy said.

"Any speculation in such helps nothing but causes unnecessary irritation."

The report also found that the militia operates from 10 ports in China's Guangdong and Hainan provinces.

Roughly 300 militia vessels are in the Spratly Islands each day, the report concluded, using remote sensing data. They fall into two categories: professional militia vessels and commercial fishing boats recruited into militia activity through "subsidy programmes", such as for fuel and navigation equipment.

"Just by being there, they normalise the idea of the Chinese presence and they deny access to fishing grounds for regional coastal states," said Mr Poling.

The AMTI report comes amid increased tensions in the South China Sea, after the Philippines said the Chinese coast guard on Tuesday fired a water cannon against boats delivering supplies to Filipino marines, forcing them to stop their work.

The Philippine boats were travelling to Second Thomas Shoal in the contested Spratly Islands.

Manila voiced "outrage, condemnation and protest", while Beijing said it acted to "safeguard China's sovereignty" as the Filipinos had not been in contact on their movements.

Washington yesterday accused Beijing of an escalation against the Philippines in the disputed waters, and warned that an armed attack would invite a US response.

"The United States stands with our ally, the Philippines, in the face of this escalation that directly threatens regional peace and stability," State Department spokesman Ned Price said in the Nigerian capital of Abuja, where he was accompanying US Secretary of State Antony Blinken on a visit.

The spokesman warned that any "armed attack on Philippine public vessels" would invoke the 1951 US-Philippines treaty in which Washington is obliged to defend its ally.