China said on Saturday a small civilian vessel from the Philippines had "illegally placed itself on the beach" of an atoll in the South China Sea that both countries claim.
The vessel landed on the Spratly Islands atoll on Friday and was there for supply purposes. The China Coast Guard was posted on the Weixin platform.
The Philippines has stationed a small number of troops on a World War Two ship grounded in 1999 as an outpost to reinforce its sovereignty claim over the Second Thomas Shoal, known as Ayungin in the Philippines and the Renai Reef in China.
Manila refused Beijing's requests last year to tow the vessel away in the row that has soured relations between the two Asian nations.
China asserts sovereignty over almost the entire South China Sea, including areas claimed by Vietnam, the Philippines, Malaysia, Brunei and Indonesia. The Permanent Court of Arbitration in 2016 China's claims had no legal basis.