(Reuters) – A Myanmar rebel alliance group has gained control of a key town along the country’s volatile northern border with China after weeks of fierce fighting with junta troops, it said in a statement late on Friday. The “Three Brotherhood Alliance”, as the group is known, said it took over Laukkai town after the military’s regional headquarters located there surrendered. The fall of Laukkai is the latest victory in a sweeping offensive by an alliance of rebel groups that began in October and has become the most significant threat to Myanmar’s military government since it seized power in a…