Beijing will start construction this year on a South China Sea islet within the Philippines' claimed exclusive economic zone as it seeks to project its power in the disputed waters, Hong Kong media reported Monday.

China will establish an outpost on Scarborough Shoal, 230 kilometres (143 miles) off the Philippine coast, the South China Morning Post newspaper cited an unnamed source close to the People's Liberation Army as saying.

Beijing claims nearly all the strategically vital sea, despite completing claims from several Southeast Asian neighbours, and in recent months has developed contested reefs into artificial islands, some topped with airstrips.

Manila claims Scarborough Shoal but says China took effective control of it in 2012, stationing patrol vessels in the area and shooing away Filipino fishermen, after a two-month stand-off with the Philippine Navy.

The SCMP cited the source as saying construction at the outpost would allow Beijing to "further perfect" its air coverage across the South China Sea, suggesting it plans to build an airstrip.

At a regular press briefing, foreign ministry spokeswoman Hua Chunying said she was not aware of the report but the area was China's "inherent territory".

Beijing will "adopt the necessary measures to resolutely protect China's sovereignty and legitimate rights and interests", she said.

The report comes ahead of an international tribunal ruling, expected within months, on a case brought by the Philippines over the South China Sea.

It also follows an announcement by the US and the Philippines that they would launch joint naval patrols in the sea.

The construction plans were likely to be accelerated in light of the upcoming ruling from the Permanent Court of Arbitration at The Hague, the newspaper cited the source as saying.

"China should regain the initiative to do so because Washington is trying to contain Beijing by establishing a permanent military presence in the region," the source said.

Washington has sailed ships close to islands claimed by Beijing, accusing China of militarising the South China Sea and deploying missiles in the area.

Hua said the recent patrol flights in the area by the Philippines and the US were "deserving of suspicion", urging "some countries" active in the region to exercise restraint and "make cooperative efforts with China".

Beijing admits building military-capable airstrips and deploying unspecified weapons on some of the islands, but insists US patrols have ramped up tensions.

As well as China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have claims to parts of the sea, which are home to some of the world's most important shipping lanes and believed to sit atop vast oil reserves.