"We call for a thorough and transparent inquiry into all aspects of the Weliweriya violence, for those conclusions to be made public, and for there to be a credible mechanism to prosecute any wrongdoing," State Department spokesperson Jen Psaki said.
"We offer our condolences to the families of the deceased and injured, continue to urge all sides to exercise restraint, and urge the authorities to respect the right of peaceful protest," she added.
"We are particularly concerned by reports that protestors seeking refuge within a Catholic church were attacked there. There is never any excuse for violence, particularly in a house of worship," Psaki said.
The government argues that the military was acting in self-defence as the protestors threw gasoline bombs.
The Sri Lankan government has announced compensation to the victims.