At least 28 people were killed by the Myanmar army at a monastery in southern Shan state, an insurgent group said.
Troops shelled Nan Naint village on Saturday, the Karenni Nationalities Defence Force (KNDF) said.
Myanmar has seen a growing number of deadly battles between its military and armed resistance groups since the junta seized power in a coup two years ago.
Some of the fiercest fighting has been in this region between the capital Nay Pyi Taw and the border with Thailand.
On Saturday, the military’s air force and artillery entered the village after the shelling around 16:00 local time (09:30 GMT) and executed villagers they found hiding inside a monastery, the KNDF said.
A video from KNDF – one of several ethnic armies which have joined the fight against the military government – showed at least 21 bodies, including three in the orange robes worn by Buddhist monks, piled up against the monastery. The bodies had what appeared to be multiple gunshot wounds. The video also shows the walls of the monastery peppered with bullet holes.
The Kantarawaddy Times, a local newspaper, quoted a KNDF spokesperson saying: “It was like the [military] made them line up in front of the monastery and brutally shot them all, including the monks.”
The group told the BBC it had found the other seven bodies nearby in the tiny village.
Some of the surrounding buildings and houses were also burned down in what the KNDF has said was a military attack on the village.
The group said the villagers had believed taking shelter with the highly-respected monks in the area might guarantee them protection. Others in the village had evacuated before the soldiers arrived.
Details of the incident are difficult to verify, but the savage nature of the attack against unarmed civilians is not new in this part of Myanmar, which has seen some of the strongest resistance to the military junta since a coup two years ago.
The KNDF told the BBC that since 25 February, there had been increased clashes and fighting as junta soldiers had advanced on the Nan Naint area and its monastery.
Nan Naint is on the main route from Shan state to Kayah state, a road the junta believes is critical to arms supply to the insurgent groups fighting against them.
It is also an area with a mixed population of sometimes rival ethnic groups: Pa-O, Shan and Karenni people.
The Pa-O National Organisation and its armed wing are strongly pro-junta in the area. Locals report the army has stepped up efforts to reinforce such pro-junta ethnic militias in the region to challenge the opposition who control the area.
And in recent months, attacks and counter-attacks had paved the way for the escalation on Saturday, observers say.
“The Karenni groups have captured some villages and so the Myanmar military is now shelling them,” a village official near the military outpost of Saung Pyaung told the local The Irrawaddy newspaper.
Such fighting has also displaced thousands of people, local aid groups report.
Myanmar’s military leaders had been hoping to hold an election this year in the belief this would give their government some badly-needed legitimacy.
But their failure to crush opposition to their rule, even with the extensive use of aerial bombardment in recent months, has made holding an election a near impossible task.
Myanmar has been caught up in a civil war for decades, which escalated after the coup in 2021.
One-and-a-half million people have been displaced, 40,000 homes have been gutted, eight million children are no longer in school, and 15 million people are judged by the UN to be dangerously short of food.
More than 2,900 people have been killed during the junta’s crackdown on dissent, according to monitoring group the Assistance Association for Political Prisoners.