Rebels assaulted the regime-controlled central prison in the main northern city of Aleppo on Wednesday, the Syrian Observatory for Human Rights said.
The mostly jihadist fighters launched an attack on the administration block of the prison, which has been under siege for months.
Signup to our newsletter and follow us on Facebook and Twitter!
Observatory director Rami Abdel Rahman said the assault, the first since the siege began, was carried out primarily by fighters from the Al-Qaeda affiliated Al-Nusra Front and from another Islamist faction, Ahrar Al-Sham.
"Fighting is going on inside the building," he said, adding that it "has not reached buildings housing prisoners."
Abdel Rahman said government aircraft were carrying out strikes on rebel positions outside the prison, located on the northern outskirts of the city.