Myanmar’s rebel forces claimed to have seized a military base near a key town at the Thailand border, in the latest blow to the ruling military regime that has been losing ground to opposition groups for months.

The Karen National Union, Myanmar’s oldest rebel group, said its military wing took control of the army base last weekend in Thin Gan Nyi Naung, a short distance from Myawaddy, a strategic trading town at the Thai border.

The fall of Myawaddy to rebel forces would be a major loss for the military regime, which has been in power for three years but slowly losing control of towns and military outposts across the country since late last year. The most significant losses have been in territories close to the China, India and Bangladesh borders.