Joint forces will begin operations in two nearby townships, the group said.
Hundreds of soldiers were injured, according to the Karen National Union.
The push in Rakhine state follows an assault on a nearby outpost that caused several hundred junta personnel to flee.
Dozens of junta troops remain stranded at a truck depot along the border with Thailand.
Those who refuse to denounce the Arakan Army are made to pay fines.
The two sides are fighting for control of Myawaddy, a major trade hub on the Thai border.
People & Power gets special access to a pivotal moment in the fight against
The Myanmar army is experiencing a rapid rise in defections and military losses, posing questions about the continued viability of the junta’s grip on power.
Airstrikes on Friday morning, residents on the border say.
More than 30 weapons were seized from the junta’s base camp, an official said.
Peace requires turning against the junta, not trying to prop up its dying power through negotiations.
Myanmar community members applied to the courts to bar the diplomat from attending the Association of Southeast Asian Nations.