The Karen National Union (KNU) has been a vocal opponent of the coup which brought the generals to power last year, and has provided shelter to dissidents working to oust the junta.

Its fighters have clashed sporadically with the military along the Thai border, sending thousands fleeing across the frontier for safety.

Friday's fighting began at 7 am (0100 GMT) in Kawkareik, which sits on the road to a major border crossing with Thailand, a resident who requested anonymity told AFP.

The fighting was ongoing, they said, adding it appeared "armed groups" were trying to attack government offices.

"We are hearing heavy weapon shelling," they said, adding that three civilians had been killed, and at least 15 had been sent to hospital.

A KNU spokesman said its fighters were battling junta troops at various locations around the town.

The military had called in multiple air strikes to support its troops on the ground, he said.

Both sides had suffered casualties, he added, without providing details.

"We are hiding in the basement," a resident of the town told AFP.

"Just in front of my eyes, I saw a vendor killed in an explosion."

Other injured people had been taken to hospital, they said.

A resident of a nearby town told AFP they had heard repeated artillery fire.

"About four jet fighters have flown across the sky many times," they said, requesting anonymity.

"We are frightened. Some people are leaving the town for safety."

Local media reported KNU fighters were clashing with junta troops at several locations around the town and that many buildings had been damaged by air and artillery strikes.

The junta's information team did not respond to a request for comment.

Myanmar has more than 20 ethnic rebel groups, many of which hold territory in the country's border regions.

Following the coup that ousted Aung San Suu Kyi's civilian government in February 2021, dozens of "People's Defence Forces" (PDF) have also sprung up to fight the junta.

PDF fighters have conducted operations alongside troops from the KNU.

In June, Thailand scrambled fighters after a junta jet briefly breached its airspace during an operation against anti-coup fighters.