As the death toll among Myanmar’s protesters rose dramatically last weekend, military airstrikes against one of the country’s largest rebel groups stoked fears of another problem: Full-fledged civil war.

The Karen National Union, which controls an area in the southeast along the Thai border, confirmed Monday that about 10,000 residents fled to a safe zone last weekend after the Myanmar military conducted airstrikes that killed three people. The fighter jets came out in retaliation for an attack by ethnic Karen rebels on a base of the national army, or Tatmadaw, in which 10 soldiers were killed and another eight were arrested.

The attacks came on the same day at least 114 people were killed in clashes with the military and police in the deadliest weekend since the Feb. 1 coup, sparking condemnation from governments around the world. With the death toll now exceeding 500 over the past two months, the prospect of a wider fight with potentially dozens of armed militias risks even more bloodshed.