The charred remains of more than 30 people, including women and children, were found in burnt-out vehicles in Myanmar on Saturday, a rebel group and a monitor said.
They accused the military government of carrying out attack.
Myanmar has been in chaos since a coup in February. More than than 1,300 people have killed in a crackdown by security forces, a local monitoring group said.
The People’s Defence Forces have sprung up in the country to fight the junta and have drawn it into a bloody stalemate of clashes and reprisals.
On Saturday, photos appeared on social media purporting to show two burnt-out trucks and a car on a road in Hpruso township in eastern Kayah state, with the charred remains of bodies inside.
A member of a local PDF group said its fighters had found the vehicles on Saturday morning after hearing the military had stopped several vehicles there in response to clashes with its fighters nearby on Friday.
“When we went to check in the area this morning, we found dead bodies burnt in two trucks. We found 27 dead bodies,“ he told AFP on condition of anonymity.
“We found 27 skulls,“ another witness said.
“But there were other dead bodies on the truck, which had been burned to pieces so we couldn't count them.“
The Myanmar Witness monitor said it had confirmed local media reports and witness accounts from local fighters. “Thirty-five people including children and women were burnt and killed by the military on 24th December Hpruso township,“ the monitor said.
Satellite data also showed a fire had occurred at about 1am local time on Friday in Hpruso, it said.
AFP did not confirm the reports. Digital verification reporters said the images purporting to show the incident had not appeared online before Friday evening.
The Myanmar military denied the claims.
A junta spokesman, Zaw Min Tun, said a clash had broken out in Hpruso on Friday after its troops tried to stop seven cars that were driving in a “suspicious way“.
Troops had killed people in the following clash, he told AFP news agency.
PDF groups have surprised the army with their effectiveness, analysts have said, as the military struggles to break resistance to its rule.
Earlier this month the US said it was “outraged by credible and sickening reports“ that Myanmar troops had seized 11 villagers, including children, in the Sagaing region and burnt them alive.
Win Myat Aye, one of a group of politicians ousted in the coup, condemned the atrocity.
“This is a cruel present from the military to our people on Christmas Day,“ he said.