Beijing started sending vessels to Thitu, part of the Spratly chain, according to the Asia Maritime Transparency Initiative (AMTI) run by the Washington-based Centre for Strategic and International Studies.
The fleet of ships, dispatched from the nearby Subi Reef, includes vessels from the navy and coastguard along with dozens of fishing boats.
The report said their presence was part of an effort to coerce the Philippines into stopping the work on the island, which China also claims.
The report said the Chinese warship was just over seven nautical miles away from the Philippine navy’s frigate the BRP Ramon Alcaraz at the time.
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The Philippine government announced in April 2017 that it would start building a beaching ramp on Thitu, which is known as Pagasa in the Philippines and Zhongye island in Chinese.
Once completed, the beaching ramp will allow Philippine ships to bring construction materials to repair and lengthen the runway on the island to accommodate larger aircraft.
The work should have been finished by the end of last year, but Philippine officials said it had been delayed by inclement weather and rough seas.
But the AMTI suggested that China’s activities had also contributed to the delay.
The Philippine Defence Secretary Delfin Lorenzana told the Philippine Daily Inquirer on Monday that the ramp was now expected to be finished by the first quarter of this year.
“The problem with Pagasa is that you have to bring in everything you need for its repair – steel bars, sand, gravel, heavy equipment,” Lorenzana said.
“You need a beaching ramp to bring these in … So I believe it should be finished by the first quarter of this year, this beaching ramp.”
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Lorenzana also said his country should protest against Beijing’s decision to build a rescue centre on Fiery Cross Reef, a Chinese-occupied outpost in the Spratly chain that is also claimed by the Philippines and Vietnam.
Citing satellite images, the AMTI said the number of Chinese ships in the area had risen to at least 24 on December 3, before the latest construction work began, and had risen to 95 on December 25. The number had dropped to 42 by January 26.
Lorenzana said in November that China’s ambassador to the Philippines had previously urged him to cancel the planned work.