Two US citizens were among 19 people killed in a raid on an alleged communist stronghold in the Philippines that has sparked a probe by the country’s rights commission, a government task force said.

More than 300 residents in the Toboso municipality of Negros Island fled their homes on April 19 as gunfire rang out in the remote sugar cane-farming region, the municipal disaster management office told reporters last week.

The number of people killed in the firefight, in which only one soldier was...