A general view taken on March 3, 2015 shows the northern Syrian city of Aleppo. Once Syria’s commercial hub, Aleppo has been devastated by fighting that began in mid-2012.

An attack on a Syrian intelligence headquarters in Aleppo killed at least 20 members of the regime’s security forces and 14 rebels, the Syrian Observatory for Human Rights said on Wednesday.

The attack, which began with a powerful bomb blast in a tunnel targeting air force intelligence offices in Aleppo, was aimed at seizing the building but failed,

Sporadic fighting was continuing in the area, it said, but heavy clashes had subsided.

Rebel sources and a combatant fighting on the government side confirmed that part of the building had been destroyed in the attack on the western outskirts of Aleppo.

Air Force Intelligence is widely viewed as one of the most powerful arms of the Syrian security establishment.

In a statement circulated by Nusra Front supporters on Twitter, the group said “the Mujahideen destroyed Air Force intelligence” and were fighting in the surrounding areas.

Rami Abdulrahman, who runs the Observatory, described it as a blow to the Syrian security establishment. “It’s very important, it should have been better protected,” he said.

Abdulrahman said the dead included members of the Syrian intelligence apparatus, pro-government fighters, and some of the attackers.

Aleppo, around 50 km (30 miles) south of the border with Turkey, 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 currently trying to broker a ceasefire between insurgent and government forces in Aleppo. His team headed there on Tuesday.

Government forces backed by allied militia launched a major offensive aiming to encircle the insurgent-held part of Aleppo late last month, but the offensive was repulsed.

[wpResize]