The effort is at the heart of plans by the Turkish-backed opposition to secure and govern a strip of territory that forms part of the last big rebel stronghold in Syria.
The presence of Turkish forces on the ground has helped to shield it from government attack.
Colonel Haitham Afisi, head of the National Army, says setting up the force has been no easy task over the last year.
"We are at the beginning. We face many difficulties but we are working to overcome them," Afisi told Reuters in an interview in the town of Azaz near the Turkish border.
Recently, he had to issue an order instructing fighters to stop "randomly opening fire", wear uniforms and cooperate with a newly established military police that represents "the force of law and justice and not a rival to any other faction".