Aid group Save the Children said it was suspending operations in Myanmar's strife-torn Kayah state after two staff members went missing in an attack that left at least 30 people dead, including women and children, with many bodies burnt.
The two staff were travelling to their home villages for the year-end holidays when they were caught up in the violence in the eastern state, Save the Children said in a statement late on Saturday.
"We have confirmation that their private vehicle was attacked and burned out," the statement said.
Opposition groups on Saturday blamed the military, which seized power from a civilian government in February, for the carnage on Friday near Mo So village of Hpruso town.
Junta spokesman General Zaw Mun Tun did not answer his telephone on Sunday. Reuters could not independently verify Saturday's accounts of the attack from a local resident, media reports and a local human rights group.
State media reported army troops had fired on and killed an unspecified number of "terrorists with weapons" from armed opposition forces fighting the military government.
Myanmar's junta previously said its troops had been attacked in Hpruso on Friday after its troops attempted to stop seven cars driving in a "suspicious way".
Troops had killed a number of people in the following clash, spokesman Zaw Min Tun told AFP, without giving details.
AFP was unable to confirm the reports surrounding the clash, but AFP digital verification reporters said the images purporting to show the incident had not appeared online before Friday evening.
Photos shared by Karenni Human Rights Group and local media showed the charred remains of bodies on burnt-out truckbeds.
A villager told Reuters on Saturday that he had seen 32 bodies, while Save the Children said at least 38 people were killed.
The London-based charity said it had suspended operations in Kayah and parts of neighbouring Karen state and in the Magway region.
“We are horrified at the violence carried out against innocent civilians and our staff, who are dedicated humanitarians, supporting millions of children in need across Myanmar," said Save the Children's chief executive, Inger Ashing.
In October the group said its office in the western town of Thantlang was destroyed in junta shelling that also razed dozens of homes following clashes with a local anti-junta group.
Myanmar has been in turmoil since the military on February 1 overthrew the elected government of Nobel Prize laureate Aung San Suu Kyi, who has been sentenced to four years' detention and faces multiple other criminal charges.
At least 1,375 people have been killed and more than 8,000 jailed in crackdowns on protests and armed opposition since the coup, according to a tally of the Association for Assistance of Political Prisoners.
The military government disputes those numbers and says soldiers have also been killed in clashes.