The Philippines demanded explanation from China's ambassador on Sunday regarding the increasing number of Chinese boats near the disputed Scarborough Shoal in the South China Sea.

Defence Secretary Delfin Lorenzana said a Philippines air force plane spotted more boats than usual boats, in a flotilla, on Saturday.

"There were four Chinese coast guard ships and six other vessels, including blue-coloured barges, around Scarborough Shoal," he told The Straits Times.

"The presence of many ships other than coast guard in the area is a cause of grave concern," he added.

The Chinese embassy in Manila could not be reached for any comment on this particular issue.

Although the area near the disputed Scarborough Shoal in the South China Sea is merely a few rocks, but it is extremely important to the Philippines due to its tranquil waters and rich stocks of fish.

Conflicts between China and the Philippines became more significant since an arbitral tribunal on July 12 rejected China's vast territorial claims in the South China Sea. The tribunal said no one country has sovereign rights over activity in the Scarborough Shoal, which is traditional fishing ground for the Chinese, Filipinos and Vietnamese.

But the tensions were expected to reduce after the G-20 summit that started on Sunday in the eastern Chinese city of Hangzhou.