The Philippines said Sunday more than 135 Chinese vessels were "swarming" a reef off its coast, describing the boats' growing presence as "alarming".
The Chinese boats were "dispersed and scattered" within the boomerang-shaped Whitsun Reef, which the Philippines calls Julian Felipe Reef, around 320 kilometres (200 miles) west of Palawan Island, the coast guard said.
Whitsun Reef is more than 1,000 kilometres from the nearest major Chinese landmass of Hainan island.
The Philippines said it counted 111 "Chinese maritime militia vessels" (CMM) on November 13. When the coast guard deployed two patrol boats to the area on Saturday the number had increased to "more than 135", the force said.
"No response was made to the radio challenges issued by the PCG (Philippine Coast Guard) to the CMM vessels which is now estimated to have grown to more than 135 vessels dispersed and scattered within Julian Felipe Reef," Manila's coast guard said, describing the boats' presence as "alarming" and "illegal".
The Chinese embassy in Manila did not immediately respond to requests for comment.
Beijing claims most of the South China Sea, including waters and islands close to the shores of its neighbours, and has ignored an international tribunal decision that its assertion has no legal basis.
It deploys vessels to patrol the waters, and has built artificial islands and military installations to reinforce its stance.
The Philippines, Brunei, Malaysia, Taiwan and Vietnam have also staked claims to various islands and reefs in the sea, which is believed to have rich petroleum reserves deep beneath its waters.
The coast guard released images Sunday which it said showed Chinese vessels lined up in formation while others were scattered around the waters.
In 2021, a similar incident involving more than 200 Chinese vessels at the reef sparked a diplomatic row between Manila and Beijing.
At the time, Manila insisted their incursion into the Philippines' exclusive economic zone was unlawful.
But China insisted they were fishing boats sheltering from bad weather and were allowed to be there.
The Philippines announced Friday that it was establishing a coast guard station on the largest island it holds in the South China Sea to improve the monitoring of Chinese vessels.
The coast guard station would be equipped with "advanced systems", including radar, satellite communication, coastal cameras and vessel traffic management, National Security Adviser Eduardo Ano said during a visit to Thitu Island.
The station has been built and is expected to be operational early next year.
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