Thai police say the latest violence was triggered when Karen groups launched an attack against Myanmar troops.

Witnesses on the Thai and Myanmar sides of the border also reported that they heard explosions and heavy machine-gun fire near a strategic bridge from late Friday that continued into early Saturday.

Fighters from the Karen National Union (KNU), the ethnic armed group that has been leading the attack on Myawaddy, last week captured the last of the Myanmar army’s outposts in and around the border town that is connected to Thailand by two bridges across the Moei River.

The latest clashes were triggered on Saturday morning when Karen groups launched an attack against Myanmar troops who were hiding near the 2nd Thai-Myanmar Friendship Bridge, a central crossing point for trade with Thailand, said police chief Pittayakorn Phetcharat in Thailand’s Mae Sot district.

He estimated that about 1,300 people fled into Thailand.

Pittayakorn said local authorities were searching those who had crossed over for weapons.

“We have given them food and moved them to the safer place,” he added.

Thai broadcaster NBT said in a post on X that the armed groups used 40mm machine guns and dropped 20 bombs from drones to target an estimated 200 soldiers who had retreated from a coordinated assault on Myawaddy and army posts since April 5.

On Saturday morning, a Thai army unit stationed on the border said clashes were ongoing with forces targeting the soldiers under the bridge.

“Currently in the middle of fighting, no losses known,” the Rajamnu Special Task Force posted on Facebook.

Thai Prime Minister Srettha Thavisin said he was closely monitoring the situation and his country was ready to provide humanitarian assistance if necessary.

“I do not desire to see any such clashes have any impact on the territorial integrity of Thailand and we are ready to protect our borders and the safety of our people,” he said on X.

Myanmar’s military, which seized power from the elected government of Aung San Suu Kyi in 2021, is facing a huge setback after losing the trading town of Myawaddy.

Since last October, the army has suffered a series of unprecedented defeats, losing large areas of territory, including border posts, to both ethnic fighters, who have been fighting for greater autonomy for decades, and pro-democracy units that took up arms after the military takeover.

Myawaddy is especially important to the military, with more than $1.1bn worth of trade passing through the town from 12 months to April.