The Philippines ordered its military to boost its presence in the South China Sea on Thursday, citing a "threat" to a garrison in the disputed waterway after nearby Chinese "activities".

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 defense department said Thursday.

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 defense 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".

Officials did not specify the nature of the "monitored Chinese activities".

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

Sandy Cay lies seven kilometres (4.3 miles) west of Thitu, where the Philippines runs an airstrip along with military and coastguard detachments. A small civilian community also live there.