The Philippines has asked China to explain the increased presence of Chinese vessels near the disputed Scarborough Shoal in the South China Sea, the defence secretary said Sunday, expressing "grave concern".

Philippine Defence Secretary Delfin Lorenzana said a Philippine air force plane saw four Chinese coast guard ships, two barge-like vessels and two suspected troop ships near the shoal on Saturday.

"The presence of so many ships, other than coast guard in the area is cause for grave concern," Lorenzana said.

"The DFA (Department of Foreign Affairs) has already called the attention of the Chinese Ambassador and demanded explanation," he added.

The shoal, which is located just 230 kilometres (650 miles) from the main Philippine island of Luzon, has long been a bone of contention between the two countries.

Lorenzana said that earlier this year, the Chinese tried to bring in dredging barges in an apparent attempt to turn the Scarborough into an artificial island but were dissuaded by the United States.

"If they try to construct anything in Scarborough it will have far reaching adverse effect on the security situation," he added.

The Chinese Embassy in Manila could not be reached for comment.

Philippine President Rodrigo Duterte has said he intends to ask Beijing -- possibly at a regional summit in Laos this week -- if they are building up the shoal despite an international court ruling rejecting most of China's claims in the resource-rich area.

Duterte said Friday he had received an "unsettling" intelligence report showing China had sent barges to the contested Scarborough Shoal and had appeared to begin construction in the area for the first time.

A UN-backed tribunal ruled in July that China's claims to almost all of the strategic sea had no legal basis and its construction of artificial islands in disputed waters was illegal.

The United States has warned of "actions" if Beijing extends its military expansion to the Scarborough Shoal.

China has sought to assert its claims in the South China Sea by building a network of artificial islands capable of supporting military operations.

Its massive land reclamation has prompted criticism from the US and claimant countries, with Washington warning it endangers freedom of navigation in international waters.

Vietnam, Malaysia, Brunei and Taiwan also have claims to the sea, through which over $5 trillion in annual trade passes.

The Scarborough Shoal, a rich fishing ground within the Philippines' exclusive economic zone and 650 kilometres away from the nearest major Chinese landmass, is a particular flashpoint.

China took control of Scarborough Shoal in 2012 after a stand-off with the Philippine navy.

Duterte, who took office in June, had earlier vowed to mend ties with China after his predecessor Benigno Aquino angered Beijing by filing the arbitration case in 2013.