Syrian army defectors have attacked a major military base near Damascus, a Syrian opposition group says. Parts of the Air Force Intelligence building in Harasta were destroyed in the overnight attack, said the Syrian Revolution General Commission. The Free Syrian Army used rockets and machine-guns, it said, in its highest-profile attack since protests began. It comes as the Arab League prepares to discuss its response to the crackdown on anti-government unrest in Syria. The Syrian government has severely restricted access for foreign journalists, and reports of violence are extremely difficult to verify. The UN says more than 3,500 people have died since protests started in March. The Syrian authorities blame the violence on armed gangs and militants. Army defectors have carried out a number of attacks on military targets in recent weeks but the Harasta incident would be their most audacious so far. Such an attack would be significant because Syria's Air Force Intelligence is one of the most feared state agencies and has been involved in the suppression of protests against President Bashar al-Assad. The base is also very close to the capital, which has remained relatively quiet in the unrest so far. 'Several explosions' The Revolution General Commission said in a statement that the defectors had attacked the building from three sides. Helicopters were also reported to have been hovering around the area. A local resident told Reuters: "I heard several explosions, the sound of machine-gun fire being exchanged." The Free Syrian Army was formed by defectors opposed to Mr Assad a few months ago, and claimed to have 15,000 members by mid-October. Its stated aim is to work alongside the Syrian people to topple the government and "stand up to the irresponsible military machine which is protecting the regime". The army says more soldiers and officers are defecting every day but acknowledges it is not strong enough to take on the Syrian army. November appears to be the bloodiest month of the eight-month revolt, with well over 300 people killed so far. Mr Assad is coming under increasing international pressure to end the bloodshed. The Arab League has already voted to suspend Syria's membership, and will ratify the action at its meeting in Morocco on Wednesday. Foreign ministers will also discuss other ways of increasing pressure on Mr Assad's regime. Syria, which will not attend the meeting, has condemned the suspension as "shameful and malicious", accusing other Arab countries of conspiring with the West to undermine the regime.