"Recently, several Philippine ships... illegally gathered under the pretence of fishing in the waters near Houteng Reef of China's Nansha Islands," China Coast Guard spokesperson Liu Dejun said in a statement, using the Chinese names for the Iroquois Reef and Spratly Islands.

"The Chinese Coast Guard took necessary control measures against the Philippine ships in accordance with the law," Liu said.

"We warn the Philippine side to immediately stop its infringements and provocations."

Meanwhile, the Philippine Coast Guard (PCG) said a Chinese navy helicopter with tail number 65 had "harassed" Philippine fishing vessels in Iroquois Reef.

The coast guard shared video footage taken by a Filipino fisherman on November 27 purportedly showing a helicopter hovering over a Philippine fishing boat.

"As far as the coast guard is concerned, this is the first time we saw a PLA Navy helicopter harassing Filipino fishermen with such low altitude, 15 to 18 feet," Philippine Coast Guard spokesman on the South China Sea Commodore Jay Tarriela told reporters.

Tarriela said China's "harassment" prompted the PCG to deploy a 97-metre and 44-metre vessel in the area "to ensure the safety and security of Filipino fishermen exercising their right to fish freely in Rozul (Iroquois) Reef."

China claims almost all of the South China Sea, despite an international ruling in 2016 concluding its claims have no legal basis.

Beijing has deployed navy, coast guard and so-called maritime militia forces -- allegedly Chinese fishing vessels -- in a bid to bar the Philippines from strategically important reefs and islands in the South China Sea.

Vessels from the two sides have clashed frequently in the past year, resulting in injuries and damages.

Tensions escalated in November when Philippine President Ferdinand Marcos signed two laws defining the country's sea waters and imposing fixed lanes for foreign ships.

That sparked a sharp riposte by China, which summoned Manila's envoy "to lodge solemn representations" over their passage.

Tensions rose again when the United States and the Philippines signed a security agreement that allows the two sides to share classified information.

Earlier on Monday, Marcos said a Russian attack submarine was spotted off the country's coast in the disputed South China Sea, describing its presence as "very worrisome".