While global attention has been fixated on the North Korean nuclear crisis over the past year, China has further fortified its man-made outposts in the disputed South China Sea in a bid to create "fully functioning air and naval bases," a think tank said Thursday.

Citing satellite images, the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies in Washington said in a report that Chinese activity in the contested waterway had involved work on facilities covering 29 hectares (72 acres) in the Spratly and Paracel chains, including islands in which Beijing is involved in territorial disputes with neighbors.

"International attention has shifted away from the slow-moving crisis in the South China Sea over the course of 2017, but the situation on the water has not remained static," the report said. "While pursuing diplomatic outreach toward its Southeast Asian neighbors, Beijing continued substantial construction activities on its dual-use outposts in the Spratly and Paracel Islands."