A high-stakes bid by Burma to bring to heel the ethnic militias which have challenged its rule for more than 60 years has driven up to 30,000 refugees into China and drawn a rare rebuke from the regime’s most important ally.

Fighting erupted last week between government troops and members of a militia known as the Myanmar National Democratic Alliance Army in the Kokang region of north-east Burma. The clashes broke a ceasefire which the government and the militia signed over 20 years ago.