When an alliance led by three rebel armies seized swaths of territory near Myanmar's border with China from the military junta last October, Beijing looked the other way.

A year on, rebel forces have ground down the junta, pushing the military out of vital borderlands and making inroads into the contested heart of Myanmar.

In response, China has sealed the border and shut off key imports to territory under rebel control, said a rebel leader and five border-area residents, a move analysts say aims to dissuade the alliance from further advances, including attacking the cultural capital of Mandalay.