It comes two days after media reported that Beijing has begun reclaiming more land in the contested Spratly Islands, a major archipelago in the South China Sea that hosts military installations by a number of countries.

“Any encroachment in the West Philippine Sea or reclamation on the features therein is a threat to the security of Pagasa Island,” the Philippine defence department said.

Manila refers to the waters immediately west of the Philippines as the West Philippine Sea, while Pagasa Island, the second biggest in the Spratlys, is also known as Thitu Island.

The defence department said it had directed the armed forces “to strengthen the country’s presence in the West Philippine Sea, following the monitored Chinese activities close to Pagasa Island”.

The agency did not specify the nature of the “monitored Chinese activities”, but the Philippine military said its air and naval patrols had observed “China militia vessels” in these areas.

Beijing claims almost all of the resource-rich South China Sea, through which trillions of dollars in trade passes annually.

Along with the Philippines, rival claimants are Vietnam, Malaysia, Brunei and Taiwan. China has ignored a 2012 ruling from a UN-backed tribunal that its claim is without basis.

In recent years it has built artificial islands on reefs while constructing military facilities and airstrips. Bloomberg reported Tuesday, citing satellite images from US officials, that new land formations have emerged in the Spratlys, on Eldad Reef, Whitsun Reef, Lankiam Cay and Sandy Cay. China’s foreign ministry called the report “completely groundless”.

A Philippine military statement said Thursday its regular air and naval patrols have “observed the persistent presence of China militia vessels around areas stated in the Bloomberg article”. The statement did not say if the patrols monitored the reported reclamations themselves.

“WESCOM (the Philippines’ Western Military Command) shall continue conducting maritime patrols and sustaining our presence in the WPS (West Philippine Sea) to protect our nation’s territory and sovereign rights,” it said.