The Philippine Navy issued four radio challenges to Chinese vessels during a maritime patrol in the West Philippine Sea after a Chinese research vessel was observed towing a floating structure inside Bajo de Masinloc, the Philippine Coast Guard (PCG) said Monday.

PCG spokesperson for the West Philippine Sea Rear Adm. Jay Tarriela said the challenges were made by BRP Diego Silang during the patrol.

"This morning, the BFAR aircraft conducted a routine MDA flight and documented the Chinese research vessel Yue Zhan Yu Ke 6 towing the floating platform back inside through the southeast entrance," Tarriela said in a social media post.

"Notably, we also documented the service boat once again repositioning the floating barrier after the CRV had re-entered," he added.

Tarriela said a People's Liberation Army Navy frigate with bow number 555 and a China Coast Guard vessel were also monitored in the area during the patrol.

When the Bureau of Fisheries and Aquatic Resources aircraft departed Bajo de Masinloc on Monday afternoon, the floating platform remained at the aft of the Chinese research vessel Yue Zhan Yu Ke 6, while Xia Yu Zhi 20028 was positioned alongside it.

The PCG earlier reported that the floating structure had been moved from the entrance of Bajo de Masinloc to inside the shoal's lagoon.

According to Tarriela, the movement was first monitored on 31 May and was supported by two boats. Maritime patrols also detected a buoy, an unidentified floating object and an antenna structure within the vicinity.

The first indication of the floating structure was recorded through satellite imagery on 25 May. A maritime domain awareness flight conducted by the Bureau of Fisheries and Aquatic Resources verified its presence near the southeast entrance of Bajo de Masinloc on 26 May, while a subsequent PCG flight on 28 May confirmed the structure.

The latest patrol indicates that the floating platform remains inside the lagoon of Bajo de Masinloc, also known internationally as Scarborough Shoal, in the West Philippine Sea.