Beijing may be prompted to fortify its defences in the disputed South China Sea, observers have warned, after new satellite images showed Vietnam had stepped up island-building in the Spratly Islands.

Vietnam occupies the most features on the contested Spratlys, which China claims as the Nansha Islands. Rival claimants over all or part of the archipelago include Malaysia, the Philippines and Brunei.

Satellite images released on August 22 by the US think tank Centre for Strategic and International Studies (CSIS) showed all 21 Vietnamese-occupied features had been expanded to include artificial land.

Vietnam is on track to surpass China in Spratlys land reclamation efforts, according to the CSIS Asia Maritime Transparency Initiative. It said 11 of these features were only the site of pillboxes, or small concrete structures commonly used by militaries, when Vietnam launched its reclamation drive four years ago.

Infrastructure such as munitions storage was taking shape on reefs where dredging activities were nearing completion, CSIS said.

Maritime analysts said the ongoing work signalled Vietnam’s resolve to solidify its claims, which would offset some of the strategic advantages Beijing secured during its own earlier wave of artificial island construction.

While the Vietnamese actions were not likely to alter the strategic equation, they might spur Beijing to upgrade its own defence posture, the observers cautioned.