A mother and her two-year-old daughter were among the dead, it was reported.
Local media said that the military destroyed two churches in two villages inhabited largely by ethnic Karen on Thursday.
According to the Karen Women’s Organisation and the Free Burma Rangers, the dead from the air strikes on villages in Karen state’s Mutraw district also included the pastor of a Baptist church, a Catholic deacon and a church layman.
Another woman and her child were wounded in a second village, they said.
“Air strikes are killing civilians and destroying homes, medical centres, churches, schools, libraries, and monasteries,” the Karen women’s group said in a statement.
The Karen live largely in the eastern part of the country along the border with Thailand and are one of the most prominent ethnic minority rebel forces that have been fighting for greater autonomy for decades.
The Free Burma Rangers said their volunteers watched from a distance as jets made two bombing runs on Thursday over Lay Wah, one of the attacked villages in Karen state’s Mutraw district, also called Papun.
The bombing forced hundreds of people to flee from their homes.
“This is totally a war crime,” Karen National Union spokesperson Padoh Saw Taw Nee was quoted by The Irrawaddy as saying.
“It is very important to stop the supply of fuel for the junta military’s aircraft. Thus, I ask again that the international community take more effective action against the junta,” he added.
The Southeast Asian country has been in the middle of a bloody conflict since the military wrested power from the elected Aung San Suu Kyi’s civilian government in a coup in February 2021. Since then, the country has been ruled by general Min Aung Hlaing, who is the commander-in-chief of the Tatmadaw, or military.
His nearly two-year rule has been marked with allegations of blatant human rights abuse and killings of political dissidents.
At least 460 innocent civilians, mostly children, have lost their lives due to the military's repeated air strikes since its takeover, said the National Unity Government, an underground umbrella group of opposition, that calls itself the country's legitimate government.
The junta, during the first year of rule, reportedly destroyed more than 100 Buddhist and Christian religious buildings in resistance strongholds in the country’s northwest, heartland and southeast regions.
In the last week of December, the military killed at least eight people and captured more than 20 civilians last week in a raid on Ah Lel Sho village in Khin-U Township, Sagaing Region.
To mark Myanmar’s 75th independence day, the military on 4 January, released 7,012 prisoners under amnesty.