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(CNN) —  

The Philippines government has filed a diplomatic protest with China over the presence of hundreds of Chinese vessels near a Philippines-administered island in the South China Sea.

As many as 275 Chinese ships and boats have been spotted this year around Thitu island, also known as Pag-asa island in the Philippines, said Jason Ramon, chief of public affairs for the Philippine military’s western command, to CNN Philippines.

Armed forces spokesperson Edgard Arevalo cautioned that it was difficult to quantify how many ships are around the island at any one time, as Chinese vessels “come and go” from the area. “The information that we got is the more likely intention is to establish their presence in the area,” he told CNN Philippines on Monday.

Philippines presidential spokesperson Salvador Panelo said Manila had several questions for Beijing about the presence of so many vessels.

“We will ask them first why. First, if they acknowledge such fact as determined by us. Number 2, ask them why they are doing it. Number 3, we will politely ask them not to … not to do what they are doing,” Panelo said, according to CNN Philippines.

China’s ambassador to the Philippines, Zhao Jianhua, said Beijing was trying to verify how many vessels were around Thitu. They might be “unarmed” fishing vessels, he said, according to CNN Philippines.

Thitu island is in the disputed Spratlys chain, which includes Chinese-controlled atolls and reefs that Beijing has built up and developed with buildings, ports and runways over the past several years.

Philippines opposition lawmaker Gary Alejano said Saturday that the Duterte administration should not let China establish any long-term presence around Thitu.

“China is establishing a norm there which we cannot alter later on. Every day is a wasted opportunity to assert our sovereignty if we do not act now,” he said, according to CNN Philippines.

Besides the Philippines, China, Taiwan and Vietnam have also laid claim to Thitu. It has a civilian population of more than 100, as well as some military personnel, according to the Asian Maritime Transparency Initiative.

The island is about 500 kilometers (310 miles) from Palawan, one of the main Philippine islands, which will be hosting exercises between the US and Philippine militaries in the next two weeks.

Manila has been in an awkward dance between Beijing and Washington for years over the South China Sea.

Under the previous government of Benigno Aquino, Manila won a landmark case against China at an international tribunal, which ruled that many of Beijing’s territorial claims in the South China Sea were unlawful.

However, the administration under current President Rodrigo Duterte has moved closer to Beijing, even as the president voiced concerns about China’s territorial ambitions.