China claims almost the entirety of the resource-rich sea, and was accused by the United States this week of efforts to "intimidate and provoke others" by parking its vessels near Whitsun Reef.

Manila has ordered Beijing to recall 183 boats at the boomerang-shaped reef around 320 kilometres (175 nautical miles) west of Palawan Island, describing their presence as an incursion of its sovereign territory.

Around 220 boats were detected by the Philippine coast guard on March 7 but only made public last weekend. A military aerial patrol over the reef on Monday found 183 were still there.

China says the fishing boats are sheltering from poor weather near the reef, which it claims is part of the contested Spratly Islands.

A spokesman for the Armed Forces of the Philippines said the additional navy ships would carry out "sovereignty patrols" in the waterway.

He did not say if the ships would go near the reef or what type of vessels would be used.

The diplomatic row has escalated with several countries, including Canada, Australia and Japan, expressing concern over the renewed tensions in the region.

Beijing often invokes the so-called nine-dash line to justify its apparent historic rights over most of the South China Sea, parts of which are also claimed by Taiwan, Malaysia, the Philippines and Brunei.

China has ignored a 2016 international tribunal decision that declared its assertion as without basis.

Philippine-China relations have warmed under President Rodrigo Duterte, who has pursued greater economic cooperation with Beijing.

He has repeatedly said conflict with China would be futile and that the Philippines would lose and suffer heavily in the process.

Duterte met with the Chinese ambassador to the Philippines this week and expressed concern over the presence of the vessels, presidential spokesman Harry Roque said Thursday.

But Roque said there was "no real controversy since they (the Chinese) are not insisting to stay there permanently."