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.

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.

- 'Right to defend themselves' -

Myanmar has been in turmoil since civilian leader Aung San Suu Kyi's ouster in the February 1 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.

Padoh Saw Taw Nee said he had expected retaliation, largely due to the KNU's condemnation of the junta and its sheltering of hundreds of anti-coup protesters in their territory.

"We already understood that they will take revenge one way or another," he told AFP.

The group released an open letter to junta leader Min Aung Hlaing this week, refusing to meet him until the military agrees to "withdraw from active engagement in politics".

After Saturday's bloodshed, Padoh Saw Taw Nee said the number of fleeing dissidents -- originally expected to reach 7,000 by April's end -- will likely be greater.

Declining to be drawn on whether its militia is planning a large-scale attack on junta forces, Padoh Saw Taw Nee only said, "if (the junta) keeps doing like this, people will have the right to defend themselves and protect themselves."

There are currently already more than 7,000 ethnic Karen people pushed into KNU-controlled territory, displaced from ongoing fighting since December.