China has built more infrastructure on artificial islands in the disputed South China Sea in the past year, including underground tunnels and radar installations, even as it has sought to repair relations with other claimants, according to an analysis of fresh satellite images.

Beijing’s construction of the seven islands in the Spratlys archipelago since 2014 has raised deep concern in the U.S. and much of Asia that China could use them to try to enforce its claims to almost all the South China Sea, one of the world’s busiest shipping routes.

China completed land reclamation in the Spratlys in early 2016, but continued such work to expand outposts in the Paracel Islands until mid-2017, according to the Asia Maritime Transparency Initiative, or AMTI, which released the images.