About 3,000 villagers from Myanmar’s southeastern Karen state fled to Thailand on Sunday following air attacks by the army on an area held by an ethnic armed group, an activist group and local media said.

Myanmar’s military launched air strikes on five areas in Mutraw district, near the border, including a displacement camp, the Karen Women’s Organization said.

“At the moment, villagers are hiding in the jungle as more than 3,000 crossed to Thailand to take refuge,” a statement said.

Thai PBS reported about 3,000 had reached Thailand.

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. 

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.

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.

AFP reported that by around 7:30 pm (1300 GMT), fighter jets had swooped into the KNU-held Hpa-pun district headquarters and opened fire on villagers, the rebel group's head of foreign affairs said.

"We can't make contact with 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.

The head of an ethnic rights group, the Karen Women's Organisation, said she lost contact with the district  soon after the jets were first sighted around 5:00 pm. 

"People were worried and scared then," Naw K'nyaw Paw told AFP, explaining that Karen state hasn't seen air strikes in years.

"But we can't get in touch with people anymore.

" She added the attack would likely fan growing fears of an impending civil war, which could spark an exodus to neighbouring Thailand.

About 90,000 refugees from Myanmar have lived in limbo on the Thai side of the border for many years after fleeing decades of conflict between Myanmar's military and ethnic armed groups.