Martin Griffiths, the under secretary general for humanitarian affairs and the emergency relief coordinator, said on Twitter that he was “horrified” by reports of the attack. “I condemn this grievous incident and all attacks against civilians throughout the country, which are prohibited under international humanitarian law,” he said.

The killings took place about 150 miles southeast of Myanmar’s capital, Naypyidaw, near a military-controlled area of Kayah State where the Karenni ethnic group has long sought autonomy from the central government.

For decades, the military has battled intermittently with several armed Karenni groups, one of the many conflicts it has waged against ethnic militias around the country. Since the coup, the fighting in Kayah State has escalated.

Save the Children said that at least 38 people had been killed in the episode on Friday morning, which took place near the village of Moso in the Hpruso township, a Christian community. The National Unity Government, a shadow administration formed by elected officials ousted after the coup, put the number of dead at 35 to 40.

According to the unity government, troops were conducting a “clearance operation” in the township, a brutal method that the military has often used to drive residents from an area, including during the 2017 campaign against Rohingya Muslims. It has been widely characterized as ethnic cleansing. In such operations, villagers are often killed indiscriminately and their homes burned.

On Friday, according to the National Unity Government, troops blocked the road outside Moso, trapped residents who were fleeing in cars and tractors, bound them and burned them alive, in what the shadow government called a “Christmas massacre.”

Four members of the junta’s border guard force, who were in the area, tried to intervene and negotiate the release of the villagers, but the soldiers shot and killed them, according to the unity government and the Karenni Nationalities Defense Force, one of the armed groups operating in Kayah State.