Satellite images show China is building an island on a reef in the disputed Spratly Islands large enough to accommodate what could be its first offshore airstrip in the South China Sea, a leading defence publication reported on Friday.

The construction has stoked concern that China may be converting disputed territory in the mineral-rich archipelago into military installations, adding to tensions over waters also claimed by Taiwan, Malaysia, the Philippines, Vietnam and Brunei.

IHS Jane's, a leading defence publication, said images it had obtained showed the Chinese-built island on the Fiery Cross Reef to be at least 1.9 miles long and 660-980ft 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.