Beijing has nearly completed building a 3,000-metre (9,800-foot) airstrip on a reef in disputed waters in the South China Sea, where tensions are mounting with its neighbours, a US think-tank said.

A satellite picture taken on Sunday showed that China was paving and marking the runway on Fiery Cross Reef and an apron and taxiway have been added, the Center for Strategic and International Studies (CSIS) said on its website.

Beijing's project to build artificial islands and facilities on various reefs and outcrops in the Spratly islands only became publicly known in recent months but construction has since been rapid, raising tensions with both its neighbours and Washington.

The South China Sea is home to strategically vital shipping lanes and is believed to be rich in oil and gas.

Washington is concerned China's efforts carry a military dimension that could undermine America's naval and economic power in the Pacific, and has weighed sending warships and surveillance aircraft within 12 nautical miles -- the normal territorial zone around natural land -- of the new artificial islands.

A lake in the middle of Fiery Cross Reef has been filled in and it has a partially-developed port with nine temporary loading piers, CSIS said.

Personnel could be seen walking around and two helipads, up to 10 satellite communications antennas and one possible radar tower were also visible, it added.

Washington wants Beijing to halt construction and militarisation, which "the Chinese show no indication of willingness to do", Bonnie Glaser of CSIS said.

She anticipated a short-term lull in construction as summer is typhoon season in the South China Sea while China's President Xi Jinping is due to visit the US in September and "the Chinese are attaching priority to having a successful summit".

But she expected activity would pick up again later.

The runway will be long enough for the People's Liberation Army to land any of its aircraft on the island, analysts say.

Arthur Ding, an expert on China's military at Taiwan's National Chengchi University, said the airstrip would "definitely improve or enhance somewhat China's military capability in the South China Sea", including being able to deploy jet fighters to the island, although they would need "sophisticated logistical" support.

Other neighbouring countries have also built artificial islands in the area, he pointed out. But the speed and scale of China's works were much faster, he told AFP, and whatever it does "definitely will have a serious repercussion on the South China Sea and the regional order".

- 'Occupied islands' -

Fiery Cross Reef, once little more than coral, is now 2.74 square kilometres (1.06 square miles) in size, Washington-based CSIS said. China has reclaimed land on seven different reefs totalling an estimated 12.8 square kilometres, it added.

At one of the sites, South Johnson Reef, CSIS said Beijing has added a small port with two loading stations, two helipads on the reef and up to three satellite communications antennas.

It also had a "large multi-level military facility" with two possible radar towers being built, along with up to six security and surveillance towers, and four possible weapons installations.

Beijing claims almost the whole of the South China Sea, including areas close to the coasts of other littoral states, locking it into disputes with several neighbours, particularly the Philippines and Vietnam.

It also has a long-running row with Japan over islands in the East China Sea.

China's foreign ministry said this week that some of the land reclamation works in the Spratlys had been recently finished on schedule, and facilities would be built mainly for civilian purposes, but "necessary military defence requirements will also be fulfilled".

Ministry spokeswoman Hua Chunying said at a regular briefing on Thursday that she was "not aware" of details of the latest CSIS report.

In a commentary Thursday China's official Xinhua news agency said that Beijing remained committed to dialogue and the peaceful settlement of disputes, stressing that it had refrained from forcefully taking back "occupied islands".

But it warned: "Expecting China to sit idly by as other countries rush to occupy South China Sea islands is unrealistic and unthinkable."