The U.S. and British embassies in Myanmar expressed concern about reports of fierce government attacks on a town in western Chin state, where the ruling junta declared martial law because of armed resistance to military rule.

The fighting began around 6 a.m. Saturday when government troops reinforced by helicopters began shelling the western part of the town of Mindat, destroying several homes, said a spokesman of the Chinland Defence Force. It is a locally formed militia group opposed to the February coup that ousted the elected government of Aung San Suu Kyi.

Helicopters also took part in the attack, according to the spokesman, who spoke on condition of anonymity for security reasons.

“Mindat town is now under siege and is bracing for an all-out assault by the junta troops from air and by land,” said a statement by the Chin Human Rights Organization.

The shadow National Unity Government, set up by lawmakers who were blocked by the army from taking their seats in Parliament, warned that “within the next 48 hours, Mindat can potentially become a battleground and thousands of people are facing the danger of being displaced.” Many have already left the town of about 50,000 people, said a resident contacted by phone who was also fleeing.

The Mindat Township People’s Administration, another opposition grouping, claimed that 15 young men had been seized by government troops and used as human shields. It said at least five defenders of the town had been killed in clashes and at least 10 others wounded.

None of these details could be independently verified, but a Myanmar state television broadcast Saturday night reported that fighting was going on, and acknowledged the town’s defenders have been putting up stiff resistance against the army.

“The military’s use of weapons of war against civilians, including this week in Mindat, is a further demonstration of the depths the regime will sink to to hold onto power,” the British Embassy said on Twitter. “We call on the military to cease violence against civilians.”

The U.S. Embassy said it was “aware of increasing violence in Mindat, including reports of the military shooting civilians,” and urged that evidence of atrocities be sent to United Nations investigators.

Detailed tallies compiled by several different watchdog groups say government security forces have killed upwards of 750 protesters and bystanders as they have tried to suppress opposition to the military’s seizure of power. In April, security forces were accused of killing more than 80 people in one day to destroy street barricades that militants had set up as strongholds in the city of Bago.