Two US B52 bombers are at the centre of an escalating row over disputed islands in the South China Sea. The unarmed jets flew over the territory in defiance of a new air defence zone imposed by China.

Beijing warned it would take “defensive emergency measures” if aircraft failed to identify themselves properly in the airspace which is about two-thirds the size of Britain.

Caroline Kennedy, the new American Ambassador to Japan and the daughter of the late President Kennedy, gave her reaction in her first public address.

“Unilateral actions like those taken by China with the announcement of an East China Sea-Air defence identification zone undermine security and constitute an attempt to change the status quo in the East China Sea. This only serves to increase tension in the region,” she said.

US ally Japan’s main airlines also defied the Chinese when they passed through the zone over the disputed islands.

Some experts believe Beijing’s move was aimed at chipping away at Tokyo’s claim to administrative control over the tiny uninhabited islands.

Analysts say China’s action may have backfired by driving the US closer to Japan and helping to bolster support for Tokyo’s agenda to strengthen its military.