Dozens of Syrian security personnel and rebels were killed during an attack Wednesday on an intelligence facility in the city of Aleppo, a monitoring group said.
The attack began when a bomb was detonated in a tunnel near the Air Force intelligence building, the Syrian Observatory for Human Rights reported.
At least 20 government troops and 14 militants were killed in the attack, which was led by the al-Qaida-linked Nusra Front and other radical Islamist armed groups.
In a statement circulated by Nusra Front supporters on Twitter, the group said that "the Mujahideen destroyed Air Force intelligence" and were fighting in the surrounding areas.
Aleppo is divided between government forces and insurgent groups fighting to topple President Bashar al-Assad in a four-year-old conflict estimated to have killed 200,000 people.
U.N. envoy Staffan de Mistura is trying to broker a cease-fire between insurgent and government forces in Aleppo and establish a "freeze zone" that he hopes will lead to greater peace in the country.