The strikes mark the first such air assault against the Fifth Brigade of Karen National Union (KNU), one of the country's largest armed groups, since the military seized power in a Feburary 1 coup.

The KNU earlier Saturday had overrun a military base in southeastern Karen state, the group said, with its Facebook page displaying a cache of seized handguns and automatic weapons.

By around 7:30 pm (1300 GMT), fighter jets had swooped into the 5th brigade-controlled Hpa-pun district headquarters and opened fire on villagers, the KNU's head of foreign affairs said.

"We can't make contact to the district yet," Padoh Saw Taw Nee told AFP.

But "there is a high school, a junior college and a medical training camp" around the attack area.

The junta did not immediately comment and there was no official confirmation of any casualties.

Myanmar has been in turmoil since civilian leader Aung San Suu Kyi's ouster in the coup, triggering a mass uprising that is increasingly being quelled with lethal force by police and soldiers.

The anti-coup movement has garnered broad support across Myanmar, including among some of the country's armed insurgent groups which have for decades been fighting the military for more autonomy.

An estimated one-third of Myanmar's territory -- mostly in its border regions -- is controlled by a myriad of rebel groups, some of whom have their own militias.