Syrian rebels assaulted the regime-controlled central prison in the main northern city of Aleppo on Wednesday, the Syrian Observatory for Human Rights told AFP.
The mostly jihadist fighters launched an attack on the administration block of the prison, which has been under siege for months.
Observatory director Rami Abdel Rahman said the assault, the first since the siege began, was carried out primarily by rebels 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.
Since July last year, Aleppo’s east district has been held by the rebels, and the west has been held by forces loyal to Assad.
The 13-member Islamic Front for the Liberation of Syria, which split off from the Syrian National Council opposition force, declared Aleppo to be an independent Islamist state months ago.
Since that time, a second civil war has begun in war-ravaged Syria and now, in addition to fighting the Syrian army, the more moderate rebel groups and the Islamist extremist groups are also fighting each other.
Members of Al-Nusra and other Syrian rebels groups have committed atrocities during the Syrian civil war, including publicly beheading a Catholic priest who was accused of collaborating with Bashar Al-Assad’s regime.
There have also been numerous clashes between Arab rebel groups and Kurdish militias in the north of the country.