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The Syrian branch of al-Qaeda, al-Nusra Front, has claimed responsibility for a deadly attack on the intelligence headquarters of Syria's air force.

At least 20 members of the security forces and 14 rebels were killed in the attack at the government building in West Aleppo on Wednesday, the Syrian Observatory for Human Rights reported.

"The goal was to storm the building and to control it, but they failed," Rami Abdelrahman, the director of the UK-based monitoring group, told the AFP news agency.

Al-Nusra Front said the attack was coordinated with other rebel factions.

The fighters detonated explosives in a tunnel near the intelligence building before launching an assault that triggered fierce clashes with government forces, said the Observatory, which relies on a network of activist sources inside Syria.

The rebels faced heavy resistance from government forces supported by fighters from the Lebanese Hezbollah movement, a key backer of Syrian President Bashar al-Assad, the Observatory said.

The fighting subsided after government warplanes carried out air strikes on rebel positions, according to activists.

"Dozens of [rebel] gunmen were killed in artillery and air strikes. The situation is quiet now in the area. There are minor sporadic clashes," AFP reported, quoting a Syrian military source.

Al Jazeera could not verify the death toll or details of the attack.

The clashes were the worst reported violence in Aleppo since a coalition of opposition groups on Sunday rejected a UN plan to freeze fighting in the northern city.

Fighting in Aleppo erupted in mid-2012, and control of the city - once Syria's commercial hub - has since been divided between rebels in the east and government forces in the west.

Source: Al Jazeera And AFP