China has built hangars large enough for some of its biggest military aircraft on three contested reefs, despite a pledge by President Xi Jinping not to militarise the South China Sea.

In an analysis of activity on Subi, Mischief and Fiery Cross reefs, each of which has an operational runway, the Center for Strategic and International Studies said the hangars could accommodate up to 12 fighter jets and four larger planes on each of the Spratly Islands. While noting there was no evidence of any Chinese military deployment in the Spratlys, the CSIS concluded from new satellite images that “the rapid construction of reinforced hangars at all three features indicates this is likely to change”.

Evidence of the hangars emerged as Beijing steps up diplomatic and military activity after last month’s ruling by an international tribunal that its territorial claims in the region had no legal basis.