The Philippines says it has discovered illegally built structures on features in the Union Banks, a series of reefs in the South China Sea.
The reefs are near where Manila says a flotilla of Chinese fishing vessels, allegedly manned by militias, have gathered in recent weeks.
The country's military said the structures were spotted during maritime patrols conducted on Tuesday, but it did not give the precise location of the structures or more details as to who erected them or as to their construction, saying only their presence violated international law.
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China has been accused of using its vast fishing fleet to help assert Beijing's territorial claims throughout the 3.36 million square kilometre South China Sea, though China has dismissed accusations it operates an irregular naval force, or maritime militia, as groundless.
"The Laws of the Sea gives the Philippines indisputable and exclusive rights over the area," Philippines Lieutenant General Cirilito Sobejana said in a statement.
"These constructions and other activities, economic or otherwise, are prejudicial to peace, good order, and security of our territorial waters.
"These structures are illegal."
Among the features in the Union Banks - which the Philippines calls the Pagkakaisa Banks - is Whitsun Reef, called Julian Felipe Reef by Manila.
The reef, which is part of the Spratly Islands archipelago, is claimed by both the Philippines and China.
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The Philippines maintains that it falls inside the country's exclusive economic zone.
A 2016 ruling by a United Nations tribunal dismissed China's claim to virtually all of the South China Sea, although Beijing has refused to recognise the decision.
China has unilaterally transformed other reefs in the Spratly chain into man-made islands with substantial infrastructure and military fortifications, including missiles, runways and weapons systems.
Whitsun Reef protects a lagoon where the Philippines says more than 200 fishing boats, allegedly operated by Chinese maritime militia personnel, have massed over the past several weeks.
Manila has protested the presence of the boats with the Chinese government as a violation of its sovereignty and called for them to leave the area.
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Beijing said the Chinese vessels are fishing boats and that they were simply escaping rough seas by moving within the lagoon formed by boomerang-shaped Whitsun Reef, which Beijing calls Niu'e Jiao and claims as part of its territory.
"Due to maritime situation, some fishing boats have been taking shelter from the wind near Niu'e Jiao, which is quite normal. We hope relevant sides can view this in a rational light," Foreign Ministry spokesperson Hua Chunying said last week.
On Wednesday, the Philippine government released images and video of the Chinese vessels, which it says were made on March 27.
Philippine government images going back to March 7 show the Chinese boats in the lagoon.
