Late last month, the ethnic armed group Karen National Union (KNU) seized a military base in eastern Kayin state, killing 10 army officers. The junta retaliated with air strikes.

The KNU has been a vocal opponent of the military junta -- which ousted civilian leader Aung San Suu Kyi from power two months ago -- and said it is sheltering hundreds of anti-coup activists.

On Saturday, the KNU condemned the use of "excessive force by engaging in non-stop bombing and air strikes" from March 27 - 30, which have "caused the deaths of many people including children".

"The air strikes have also led to the further displacement of more than 12,000 people who have fled their villages and caused a major humanitarian crisis."

Junta spokesman Zaw Min Tun said the military has only been targeting KNU's 5th Brigade -- which led the seizure of the military base and killed officers.

"We had an air strike on that day only," he told AFP.

"We have signed a nationwide ceasefire agreement... If they follow the NCA, there is no reason for conflict to happen," Zaw Min Tun said.

Ethnic Karen local media and rights groups have reported multiple bombings and air strikes across the state over recent days.

About 3,000 people fled to neighbouring Thailand on Monday, crossing the Salween River to seek shelter. But most returned to Myanmar by Wednesday, which Thailand claimed was "voluntary".

Myanmar has been in turmoil since the military seized power on February 1, triggering a massive uprising across the country with protesters demanding the restoration of the elected government.

Information flow in the country has also been throttled, with the junta cutting wifi services, mobile data and imposing a nightly internet blackout that has gone on for nearly 50 days.

Myanmar's border regions are largely controlled by various ethnic armed groups that have long agitated for greater autonomy.

Territory in the northern Kachin state -- held by the Kachin Independence Army -- has also seen a recent step-up in military activity.