One of Myanmar’s main ethnic minority rebel groups warned of a growing threat of major conflict on Tuesday and called for international intervention against a military crackdown on opponents of last month’s coup.

Myanmar has been in turmoil since the army ousted an elected government led by Nobel laureate Aung San Suu Kyi on February 1, detaining her and reimposing military rule after a decade of tentative steps towards democracy.

While cities and towns across the country have been consumed by protests against the military, fighting has also flared between the army and insurgents in frontier regions and refugees are spilling over borders.

The Karen National Union rebel group, which operates in the east along the border with Thailand, said it was bracing for a major government offensive.

“Now, thousands of Burma military ground troops are advancing into our territories from all fronts,” the group said in a statement.

“We have no other options left but to confront these serious threats posed by the illegitimate military junta’s army in order to defend our territory.”

The group urged the international community, Thailand in particular, to help Karen people fleeing the “onslaught” and for countries to cut ties with the junta to stop the violence against civilians.

A spokesman for the Myanmar junta did not answer calls seeking comment.

Opponents of the coup have called for a united front with insurgent groups to help those standing up to the military.

Rebels have battled the government for decades for greater autonomy in remote border regions. The military has justified its long grip on power by saying it is the only institution capable of ensuring national unity.

Military aircraft bombed KNU fighters on the weekend, sending some 3,000 villagers fleeing to neighbouring Thailand. Thailand denied accusations from activists that refugees were being forced back but a Thai official on the border said the army was sending most people back because it was deemed safe on the Myanmar side.

A spokesman for the UN refugee agency said it was concerned about reports people were being sent back and it was seeking information from Thailand.

Fighting has also flared in the north where ethnic Kachin insurgents battle government forces.

The military seized power saying that November elections won by Suu Kyi’s party were fraudulent, an assertion dismissed by the election commission.

At least 512 civilians had been killed in nearly two months of protests against the coup, 141 of them on Saturday, the bloodiest day of the unrest, according to the Assistance Association for Political Prisoners (AAPP) advocacy group.

Despite the surge in violence, thousands of protesters came out to march in several towns on Tuesday, according to photos on social media.

Security forces shot and killed one man in the southernmost town of Kawthaung, the Mizzima news portal reported, and one person was killed in Myitkyina in the north, a relative of the 23-year-old victim told Reuters.

A civil disobedience campaign of strikes has paralysed parts of the economy and protesters stepped it up by asking residents to leave rubbish at intersections in Yangon.