Children were among those killed and injured in airstrikes by Myanmar's military on an ethnic rebel-controlled area in the country's southeast, a humanitarian aid group said Tuesday, as junta forces stepped up their offensive in the region.

Myanmar's military, which seized power in a February 1 coup, has been conducting airstrikes in southeastern Karen state since Saturday night, forcing thousands of residents to flee into the jungles.

Many of the villages targeted are controlled by the Karen National Union (KNU), an ethnic armed group that holds large swathes of territory in the borderlands.

A 5-year-old boy died in a bombing on Sunday, and a 12-year-old girl was hit in the face by bomb shrapnel, according to relief organization Free Burma Rangers (FBR). The girl had come to Day Bu Noh village to attend school, the group said.

A high school in Dwe Lo Township was destroyed in an airstrike on Monday, but no casualties were reported as the students were in hiding, FBR said. Then on Tuesday, six people were killed and 11 injured by airstrikes near Hsaw Hti township.

In a statement Tuesday, the KNU condemned the Myanmar military's bombing campaign. "Villagers, including underage children, have been killed by airstrikes," the KNU statement said. "Many are injured."

CNN has not been able to independently verify these reports.

Myanmar has been thrown into turmoil since Gen. Min Aung Hlaing ousted the elected government of Aung San Suu Kyi and installed a military junta two months ago. Protesters have taken to the streets nearly every day, and workers in various sectors including health care and transport have gone on strike, disrupting the economy.

The military has responded by embarking on a systematic and bloody crackdown, shooting unarmed civilians in the street, while carrying out beatings, arbitrary detentions and nighttime raids on the homes of suspected opposition members. This weekend saw the bloodiest crackdown yet, with at least 114 people killed on Saturday.

Residents fleeing the violence have fled to several neighboring countries, including Thailand and India.

Myanmar security forces have killed at least 521 people since the coup, with more than 2,600 detained, according to advocacy group Assistance Association for Political Prisoners (AAPP).

•Sourced from CNN.

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