Myanmar's military sparked a civil war, seizing power in a 2021 coup and now battling an array of pro-democracy guerrillas and ethnic minority armed groups which have long been active in the country's fringes.

Much of the fighting has focused on control of border crossings where combatants can fill their war chests by running lucrative toll gates controlling trade and smuggling routes.

Thailand's military said the Karen National Liberation Army and the Karen National Defence Organisation "used unmanned aerial drones to launch a bombing attack" on a junta border base around 4:45 am (2145 GMT Wednesday).

"414 displaced people from Myanmar fled across to the Thai side", near Mae La in Tak province, around 250 miles (400 kilometres) northwest of Bangkok, a statement said.

They have been housed in a nearby temple and monastery while Thai troops have stepped up border patrols, the statement added.

Armed organisations recruiting from the Karen ethnic group have been active for decades and have emerged as key challengers to the junta holding control over border crossings with Thailand.

Myanmar's civil war has caused huge waves of population displacement, with 81,000 refugees or asylum seekers from the country currently living in Thailand, according to United Nations figures.