The humanitarian group said two staff members were caught up in the attack, which took place near a village in Kayah state, as they were travelling back to their office.
Two residents and the Karenni Human Rights Group, which operates in the area, said on Sunday that soldiers had killed more than 30 civilians and set their bodies ablaze.
State media reported that soldiers had fired on and killed an unspecified number of “terrorists with weapons” in the village. It did not say anything about civilians.
In a statement on Tuesday, Save The Children’s chief executive, Inger Ashing, said: “Violence against innocent civilians including aid workers is intolerable, and this senseless attack is a breach of International Humanitarian Law.
“This is not an isolated event,” she added. “The people of Myanmar continue to be targeted with increasing violence and these events demand an immediate response.”
In February this year, the Tatmadaw, Myanmar’s military, staged a coup d’état, ousting the elected government and arresting officials including Aung San Suu Kyi.
The coup sparked international outcry, and security forces quashed nonviolent nationwide demonstrations with force, killing almost 1,400 civilians, according to the Assistance Association for Political Prisoners.
Save The Children has urged the UN Security Council to respond to the military’s violent methods with several solutions, including an arms embargo.
Photos of the Christmas Eve attack have spread online, with photos showing the charred bodies of over 30 people in three burned-out vehicles who were reportedly shot by government troops as they fled.
A senior UN official on Tuesday called on Myanmar authorities to investigate the reported killings, saying he saying he was “horrified” at the violence.
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, using Myanmar’s previous name.
Save The Children suspended its operations in parts of Myanmar after the staff members were declared missing.