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.
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.
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Washington wants Beijing to halt construction and militarization, which "the Chinese show no indication of willingness to do", Bonnie Glaser of CSIS said.
But she expected activity would pick up again later.
The island is now 2.74 square kilometres in size, 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.
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.