Two workers for non-profit group Save the Children remain missing after their vehicle was among several that were attacked and burned in the incident in eastern Kaya state.

A monitoring group and local media have blamed the attack on junta troops.

"I condemn this grievous incident and all attacks against civilians throughout the country, which are prohibited under international humanitarian law," United Nations under-secretary-general for humanitarian affairs Martin Griffiths said in a statement.

"I call upon the authorities to immediately commence a thorough and transparent investigation into the incident so that perpetrators can be swiftly brought to justice," said Griffiths.

Myanmar has been in chaos since a February coup, with more than 1,300 people killed in a crackdown by security forces, according to a local monitoring group.

"People's Defence Forces" (PDF) have sprung up across the country to fight the junta, and have drawn the military into a bloody stalemate of clashes and reprisals.

On Saturday, photos appeared on social media purporting to show two burned-out trucks and a car on a highway in Hpruso township in Kayah state, with the charred remains of bodies inside.

A member of a local PDF group said its fighters had found the vehicles Saturday morning after hearing the military had stopped several vehicles in Hpruso after clashes with its fighters nearby on Friday.