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MANILA - China's dealings with the Philippines may not be based purely on friendship, an analyst said Wednesday, after Beijing said it "reasonably" expanded its islands in waters that are disputed by Manila.

China fortified its structures in the South China Sea as it strengthened economic ties with the Philippines under President Rodrigo Duterte.

The structures, which some fear could be for military use, places the country within range of China's weapons if they are placed in the area, said University of the Philippines maritime law professor Jay Batongbacal.

• China confirms 'reasonable' expansion of S. China Sea islands

"Who was it who said, 'there are no permanent friends, only permanent interests.' We have to accept the fact that China's interest may not necessarily be fully be invested in simple friendliness. Let's not be naive," he told ANC Wednesday.

Batongbacal said the expansion in the disputed sea would leave parts of the country, particularly Visayas and the rest of the southern Philippines, exposed to the monitoring of China.

"With regards to the surveillance equipment that already gives China the ability to almost completely listen in on electronic signals within the Philippines," he said.

Presidential Spokesperson Harry Roque Jr. earlier said Manila continued to rely on China's "good faith."

Batongbacal said he hoped the interest of claimant countries would prevail in upcoming talks between ASEAN and China on a code of conduct in the disputed waters and that the Philippines would stand its ground.