The staff members were reportedly travelling to their home villages for a Christmas break on Friday in conflict-torn eastern Kayah state, the UK-based charity said.
It accused the ruling military of the violence.
A statement read: “We have confirmation that their [staff members] private vehicle was attacked and burned out. The military reportedly forced people from their cars, arrested some, killed others and burned their bodies.”
Inger Ashing, chief executive, added that attacks on aid workers are unacceptable: “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.
“Investigations into the nature of the incident are continuing but attacks against aid workers cannot be tolerated.”
The charity said it had suspended operations in Kayah, parts of the neighbouring Karen state and in the Magway region.
At least 35 charred bodies were discovered in eight burned vehicles and on five motorbikes near Mo So village, Hpruso town, on Christmas morning.
Some of the bodies were those of the elderly, women and children, according to the local Karenni Human Rights Group.
The Myanmar military, which wrested power from the elected government of Aung San Suu Kyi in February, is accused of carrying out the carnage.
Army troops had fired on and killed an unspecified number of “terrorists with weapons” from opposition forces in the village, state-run media reported.
Rebel groups in the region said the military had killed civilians.
A commander of Karenni National Defence Force told independent news agency Myanmar Now: “We don’t know exactly how many women, men and children are among those burned. Some became ashes, some others were charred. The [bodies] were no longer recognisable or identifiable when we saw them.”
Photos by the Karenni Human Rights Group showed the charred remains of several bodies lying on burnt-out trucks.
It said on social media: “We strongly denounce the inhumane and brutal killing which violates human rights.”
The south Asian country is grappling with civil strife since the junta wrested power after alleging fraud in an election that Ms Suu Kyi’s party had won. Independent observers had insisted that the ballot was fair.
Civilians have since been taking up arms in protest and have demanded the restoration of democracy.
The National Unity Government, a broad alliance of anti-coup organisations that include members of Ms Suu Kyi’s ousted ruling party, has backed the training and formation of militias called the People’s Defence Forces and has declared a nationwide rebellion against the junta.