Some 10,000 Burmese people have fled to Thailand to escape fierce fighting between the military and units of a powerful ethnic armed group since Wednesday, Thai authorities said.
They are fleeing from Shwe Kokko town, controlled by a pro-military militia and home to Chinese-owned casinos.
This is one of the largest cross-border movements of people since a military coup two years ago.
The military has not released a statement yet about the fighting.
It is the latest in a civil war that has been raging since the coup in February 2021. Two years on, the military government has failed to impose its authority on large areas of the country. It is battling established ethnic armed groups in border areas that have been at war with the military for decades, and recently-formed anti-coup militias that call themselves People's Defence Forces (PDFs) in much of the rest of the country.
Many thousands of people have been killed and some 1.4 million have been displaced since the coup. Nearly one third of the country's population is in need of aid, according to the United Nations.
The latest fighting broke out after the Karen National Liberation Army (KNLA) and its allies launched attacks on military outposts and a gate camp near Shwe Kokko on Wednesday. More than 80 people have been killed on both sides, KNLA told BBC Thai.
Aid workers in the border regions - Thailand's Mae Sot and Mae Ramat areas - have called for urgent humanitarian assistance as refugees seek shelter in schools, monasteries and rubber farms.
"In the long run, we need more donors," said Kay Thi Htwe, a Burmese volunteer at a monastery in Mae Sot, which is hosting 500 refugees.
The KNLA has also closed the Myawaddy-Kawkareik Asia highway - one of the main roads to the border - for two weeks starting Friday.
Back in Shwe Kokko, the military-aligned Border Guard Forces which controls the enclave is protecting the casinos and warning residents to stay indoors.
This comes as the military continues to crush civilian resistance, targeting schools, clinics and villages.
Earlier this week, the military said it had arrested 15 teachers who had been giving online classes for a school backed by the exiled National Unity Government (NUG).
The teachers were taken from their homes in Mandalay, Saigang and Magway, a member of the General Strike Committee of Basic Education Workers told the BBC. In July, about 30 teachers were reportedly arrested because they worked for an NUG-recognised online school.
From the start, education has been a battleground in Myanmar. Teachers were among the first, along with health-workers, to walk out in protest against the coup, and were in the front line of the huge protests called by the Civil Disobedience Movement in the first weeks after the military takeover.
When that was crushed, most still refused orders by the military junta that they should return to work, and in May 2020 around 150,000 teachers and university lecturers were dismissed from their jobs. Many decided to go underground, joining schools and clinics in areas where communities had begun an armed struggle against military rule.
The military views the establishment of independent schools and clinics as an existential threat. Official figures suggest the number of students taking the 10th grade matriculation exam in state-run schools is now only one fifth of the number doing so before the coup. Teachers working outside the state sector have been branded as terrorists.