Story highlights China is building islands in disputed waters in the South China Sea Land reclamation taking place at five sites, IHS Jane's Defence Weekly says. China creating 'chain of air and sea capable fortresses'

Satellite images from late January analyzed by IHS Jane's Defence Weekly show that China is reclaiming a significant amount of land at three sites in the Spratly Islands, in addition to two other sites previously documented by the defense publication.

"Where it used to have a few, small concrete platforms, it now has full islands with helipads, airstrips, harbors, and facilities to support large numbers of troops," James Hardy, Jane's Asia Pacific editor, told CNN.

"We can see that this is a methodical, well-planned campaign to create a chain of air and sea capable fortresses across the center of the Spratly Islands chain."

In November, the publication reported that China was building an "air strip capable" island at least 3,000 meters long on Fiery Cross Reef.

The South China Sea is the subject of numerous rival -- often messy -- territorial claims, with China, Brunei, Malaysia, the Philippines, Taiwan and Vietnam disputing sovereignty of several island chains and nearby waters.

The areas in dispute include fertile fishing grounds and potentially rich reserves of undersea natural resources.

READ: China's military not ready 'to fight and win future wars'

The latest images show for the first time reclamation at Hughes Reef -- a "large facility" is being constructed on 75,000 square meters of reclaimed land. Jane's also said that significant progress has been made at Johnson South and Gaven Reefs.

"The buildings on Hughes Reef and Gaven Reefs have almost identical footprints: that of a main square building with what appears to be an anti-aircraft tower or radome at each corner.

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Is China building new islands in disputed waters?