Myanmar’s military appears to have abandoned the strategically important border town of Myawaddy on the eastern border with Thailand, after a sustained assault by an alliance of anti-coup fighters.

Thai PBS reported on Monday that more than 600 Myanmar nationals, including 67 military officers and 410 non-commissioned officers as well as their family members, were waiting for entry to Thailand after the regime in Naypyidaw requested permission for an aircraft to land on the Thai side to collect the group.

The Karen National Union (KNU) earlier said that soldiers from its armed wing, the Karen National Liberation Army, in alliance with anti-coup fighters from the People’s Defence Force (PDF), had taken control of the military base in the town, sharing photos of weapons and ammunition they said they had seized.

Myawaddy lies just across the border from Mae Sot in Thailand and is a major trading route.

Phil Robertson, deputy Asia director for Human Rights Watch, said that even with the renewed fighting since the February 2021 coup, bilateral trade across the checkpoint between April 2023 and March 2024 amounted to more than $1bn and Myawaddy’s loss was significant.

“This is a major blow to the Myanmar military junta,” he told Al Jazeera. “This is the largest and most important of the six official Thai-Burma border crossings.”  Myanmar was previously known as Burma.

The generals led by army chief Min Aung Hlaing seized power from the elected government of Aung San Suu Kyi in February 2021, triggering a crisis that has displaced thousands and left millions in need of humanitarian assistance.

The military has come under increasing pressure across the country since the Three Brotherhood Alliance, another coalition of ethnic armed groups, launched a major offensive in northern Shan and western Rakhine states at the end of last October, seizing hundreds of military outposts.

“The military has really backed itself into a corner, waging a very unpopular war against the people of Myanmar who refuse to go back to a military dictatorship and all that that entails,” Robertson said.

Last week, the National Unity Government (NUG), a parallel government of elected politicians overthrown in the coup, said their forces carried out a drone attack on Naypyidaw, the military’s remote, purpose-built capital.

The KNU is Myanmar’s oldest ethnic armed group and was one of the first to condemn the coup.

It has provided training and expertise to members of PDFs, which were established by the NUG.