Pictures posted on social media on Christmas day show burnt-out vehicles on the highway near the village of Mo So in the state of Kayah in Myanmar.
A member of the armed resistance group PDF told the AFP news agency that its members found the vehicles - two trucks and a car - on Saturday morning after hearing that government forces had stopped several vehicles in Hpruso.
- When we went to look at the place this morning, we found burnt bodies in two trucks.
We found 27 dead, says the PDF member who does not want to be named.
Witness: The victims were tied up
Another witness told AFP that 27 dead could be counted, but that some of the bodies were so badly burned that they could not be counted.
A resident of Mo So told the AP that he went to the scene, and that the victims were civilians who fled fighting between armed resistance groups and government forces in the nearby village of Koi Ngan.
The witness further states that the victims had been tied up before they were burned.
Myanmar's military junta has not yet commented on the incident, but state-controlled media report that a number of "terrorists with weapons" were killed in the attack.
Terrified by the violence
The voluntary organization Save the Children announced on Saturday that two of the organization's employees are missing after the attack on the motorway, and that it has received confirmation that their private vehicles were attacked and burned out.
The two were on their way home after carrying out humanitarian work in the region, Save the Children writes in a statement.
"We are appalled by the violence against innocent civilians and our staff, who are dedicated human rights workers who support millions of children in need throughout Myanmar," said International Save the Children CEO Inger Ashing in a statement.