The Philippine military said on Thursday it had documented illegal manmade structures on Union Banks in the South China Sea's Spratly Islands, near areas where hundreds of Chinese vessels had massed last month.
Military chief Lt. Gen. Cirilito Sobejana said the structures were not far from islands and features claimed by the Philippines inside its exclusive economic zone.
He did not say which country had built the structures or the extent of the construction.
They were discovered on March 30, during one of the military's maritime flight patrols over the South China Sea, where it has been closely monitoring the activities of the Chinese vessels believed to be manned by militias.
"These constructions and other activities, economic or otherwise, are prejudicial to peace, good order, and security of our territorial waters," Sobejana said in a statement.
"Our efforts to dutifully perform our mandate to protect and advance our national interests in the area continue unhampered," he said.
The islands and reefs of the Spratlys are among the world's most contested features, with Vietnam, China, Taiwan, the Philippines and Malaysia putting structures, small communities and artificial islands there to stake their territorial claims in the strategic waterway.
Video footage shared by the military showed hundreds of boats spread around Hughes, Gaven and Whitsun reefs, some moored in groups of as many as seven boats side-by-side.
Calls to the Chinese embassy in Manila on Thursday seeking comment on the Union Banks structures went unanswered.
Sobejana remarks come as part of a new push by the Philippines' military and diplomats to publicly challenge China's maritime activities after several years during which criticism was muted.
Philippine President Rodrigo Duterte has sought closer relations with Beijing and has said challenging its actions are futile and dangerous.
The United States and Philippine national security advisers discussed their concerns over Chinese activities in the South China Sea in a call on Wednesday, while Philippine Foreign Secretary Teodoro Locsin is due to meet his Chinese counterpart this week.