Two Save the Children staff members were among at least 35 people, including children, who were killed in Myanmar on Christmas Eve during an attack blamed on government forces.
The charity said both men were new fathers who were "caught up" in the rampage in Kayah state while on their way back to their office after working in a nearby community.
One was 32 years old, with a 10-month-old son, and had worked at Save the Children for two years, training teachers.
The other was 28, with a three-month-old daughter, and joined the charity six years ago.
Save the Children said they are not being identified for security reasons.
It was reported last week that the military had shot dead more than 30 people and burnt their bodies.
The Karenni Human Rights Group said they discovered the charred bodies near Mo So village in the Hpruso township in the eastern state of Kayah on Christmas Day.
Save the Children chief executive Inger Ashing said: "This news is absolutely horrifying. 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."
Save the Children called on the UN Security Council to respond to the violence with steps including an arms embargo.
The group said the Association of South East Asian Nations (ASEAN) must also convene an urgent meeting to take action on an agreement made in April, which called for an immediate cessation of violence in Myanmar and mediation by the ASEAN Special Envoy.
Purported images of the aftermath of the killings went viral on social media, fuelling public outrage at the country's ruling military junta, which seized power in a coup in February.
The accounts have not been independently verified, but the pictures showed the charred remains of at least 30 people inside burned-out trucks.
A villager who went to the scene previously said the victims had been fleeing fighting between Myanmar's military and local militia groups when they were arrested and killed by soldiers.
He said the bodies were tied with ropes before being set on fire.
Myanmar's military has not commented on the allegations.