Myanmar has been in turmoil since civilian leader Aung San Suu Kyi was deposed in a February 1 coup, triggering a mass uprising across the country.

As security forces have deployed deadly violence against civilians to suppress a persistent anti-junta movement, some of Myanmar's myriad ethnic armies have spoken out against the military.

Among the most prominent opponents is the Karen National Union (KNU), which has admitted offering shelter to fleeing dissidents in the territory they control along Myanmar's eastern border.

Clashes have escalated in Karen state between the KNU's fighters and the military.

Last week the rebels attacked and razed a military base. The junta retaliated with airstrikes and rocket launchers aimed at rebel-held territory.

On Sunday -- after five days of air raids -- the vice chief of staff for the KNU's armed wing wrote an open letter calling for all ethnic Karen fighters to unite, regardless of their loyalties.

"Never has there been such a great opportunity during the 70-plus years of revolution. Take advantage of this and fight against the Burmese military dictatorship," said Lieutenant General Baw Kyaw Heh.

"In our generation, let us stand united... to escape the military dictatorship."

His letter was addressed to two other rebel groups in Karen state -- the Democratic Karen Buddhist Army and the KNU/KNLA Peace Council, a group formed by a former KNU commander. Both have remained mum since the coup.

State-run media last week reported that their officials had met with the junta in separate meetings to discuss "peace processes".

Baw Kyaw Heh also called for unity among ethnic Karen fighters in the Border Guards Forces (BGF) -- a subdivision of Myanmar's military made up of former ethnic insurgents.

"So BGF, as you are Karen, you need to think carefully about it and make the right decision," he said. "Karen people should not be killing each other."

Myanmar has more than 20 ethnic rebel groups, many of whom hold territories in the country's border regions.

A messy struggle over autonomy, control of lucrative drug production and natural resources has long pitted them against each other and the military.

Since the coup, besides fighting the KNU in the east, the junta has also engaged in artillery shelling and airstrikes against the Kachin Independence Army in the north.

Unity among the rebel groups appears an unlikely prospect due to their fighting and a long-held general distrust of Myanmar's ethnic Bamar majority.