The photos showed charred bodies in three burned-out vehicles who were reportedly shot by government troops as they were fleeing combat. The accounts could not be independently verified.
The international aid group Save the Children said that two of its staffers were missing in the massacre, which sparked outrage against the military that took power after ousting the elected government of Aung San Suu Kyi. Save the Children said it was suspending operations in the region.
On Sunday, the US Embassy in Myanmar said it was appalled by the "barbaric attack in Kayah state that killed at least 35 civilians, including women and children".
"We will continue to press for accountability for the perpetrators of the ongoing campaign of violence against the people of Burma," it said in a statement.
A villager who said he went to the scene told The Associated Press that the victims had fled the fighting between armed resistance groups and Myanmar's army near Koi Ngan village, which is just beside Mo So, on Friday. He said they were killed after they were arrested by troops while heading to refugee camps in the western part of the township.
Save the Children said two of its staff who were travelling home for the holidays after conducting humanitarian response work in a nearby community were "caught up in the incident and remain missing."
"We have confirmation that their private vehicle was attacked and burned out," the group added in a statement. "The military reportedly forced people from their cars, arrested some, killed others and burned their bodies."
The government has not commented on the allegations, but a report in the state-run Myanma Alinn daily newspaper on Saturday said the fighting near Mo So broke out on Friday when members of ethnic guerrilla forces, known as the Karenni National Progressive Party, and those opposed to the military drove in "suspicious" vehicles and attacked security forces after refusing to stop.
The newspaper report said they included new members who were going to attend training to fight the army, and that the seven vehicles they were travelling in were destroyed in a fire. It gave no further details about the killings.
Myanmar's independent media reported on Friday that 10 Mo So villagers including children were arrested by the army. The media said that four members of the local paramilitary Border Guard Forces who went to negotiate for their release were reportedly tied up and shot in the head by the military.
The witness said the villagers and anti-government militia groups left the bodies as military troops arrived near Mo So while the bodies were being prepared for cremation.
"It's a heinous crime and the worst incident during Christmas. We strongly condemn that massacre as a crime against humanity," said Banyar Khun Aung, director of the Karenni Human Rights Group.
Earlier this month, government troops were also accused of rounding up villagers, some believed to be children, tying them up and slaughtering them.Â
Fighting resumed over the weekend on the border with Thailand, where thousands of people have fled to seek shelter.
