The construction has stoked concern that China may be converting disputed territory in the mineral-rich archipelago into military installations, adding to tensions waters also claimed by Taiwan, Malaysia, the Philippines, Vietnam and Brunei.
IHS Jane's said images it had obtained showed the Chinese-built island on the Fiery Cross Reef to be at least 3,000 meters (1.9 miles) long and 200-300 meters (660-980 ft) wide, which it noted is "large enough to construct a runway and apron."
The building work flies in the face of US calls for a freeze in provocative activity in the South China Sea, one of Asia's biggest security issues. Concern is growing about an escalation in disputes even as claimants work to establish a code of conduct to resolve them.
Dredgers were also creating a harbor to the east of the reef "that would appear to be large enough to receive tankers and major surface combatants," it said.
Asked about the report at a defense forum in Beijing on Saturday, Jin Zhirui, a colonel with the Chinese air force command, declined to confirm it but said China needed to build facilities in the South China Sea for strategic reasons.
"We need to go out, to make our contribution to regional and global peace," Jin said. "We need support like this, including radar and intelligence."
The land reclamation project was China's fourth in the Spratly Islands in the last 12 to 18 months and by far the largest, IHS Jane's said.
It said Fiery Cross Reef was home to a Chinese garrison and had a pier, air-defense guns, anti-frogmen defenses, communications equipment, and a greenhouse.
Beijing has rejected Washington's call for all parties to halt activity in the disputed waters to ease tension, saying it can build whatever its wants in the South China Sea.
Hong Kong media have reported that China plans to build an air base on Fiery Cross Reef. In August, the deputy head of the Chinese Foreign Ministry's Boundary and Ocean Affairs Departments said he was unaware of any such plans.

Latest News from World News Desk