China will start reclamation at the Scarborough Shoal in the South China Sea later this year and may add an airstrip to extend its air force's reach over the contested waters, a military source and mainland maritime experts say.
A source close to the PLA Navy said Beijing would ramp up work to establish a new outpost 230km off the coast of the Philippines as the US and Manila drew their militaries closer together.
An upcoming ruling on territorial claims by the Permanent Court of Arbitration at The Hague, widely expected to go against China, would also accelerate the plan, the source said.
Manila wants the court to declare that Beijing's claims must comply with the United Nations Convention on the Law of the Sea, and the decision could come next month or in June.
"Beijing will take action to carry out land reclamation at Huangyan Island within this year," said the source, who requested anonymity, referring to the shoal.
"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 US and the Philippines began joint patrols in the South China Sea in March, US defense chief Ash Carter revealed during his latest visit to the region. US forces will also have access to at least eight military bases in the Philippines, with two air bases in Pampanga, 330km from Scarborough Shoal.
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The atoll is a potential flashpoint in the disputed South China Sea and is claimed by Beijing, Manila and Taipei. Chinese coastguard ships took control of the area after a tense stand-off with Philippine vessels in 2012.
With a new outpost in the shoal, Beijing could "further perfect" its air coverage across the South China Sea, the source said.
The PLA can already land planes at Woody Island, and two additional airstrips are believed to be under construction at Mischief and Fiery Cross reefs.
Subi Reef could also support a landing strip. Last month, the head of US naval operations, Admiral John Richardson,said Chinese activity had been observed around the shoal.
"If China finishes land reclamation at Scarborough Shoal, it can install radar and other facilities for 24-hour monitoring of the US Basa air force base on Pampanga," Macau-based military expert Antony Wong Dong said.
US defense officials have confirmed China deployed two J-11 fighter jets and bolstered its advanced surface-to-air missile system on Woody Island. Four of the eight HQ-9 launchers were operational, according to US Fox News.
Professor Jin Yongmin, director of the Ocean Strategy Studies Centre at the Shanghai Academy of Social Sciences, said an airstrip at Scarborough Shoal would extend China's air force reach in the South China Sea by at least 1,000km and close a gap in coverage off Luzon, a gateway to the Pacific. Beijing had been placed under "extreme duress" by the intensified US-Philippine cooperation and impending ruling by The Hague, Jin said.
Another driving factor was Manila's outpost at Thitu Island in the Spratly chain, Professor Wang Hanling, of the Chinese Academy of Social Sciences, said. It is home to an unpaved landing strip, which the Philippines has said it will repair, although the work allows for the facility to be upgraded.
Foreign Minister Wang Yi said in February that Beijing halted reclamation work in the Spratlys last August, but other countries continued with their projects.