Rebels have carried out a suicide bomb attack overnight on an intelligence agency compound in a suburb of the Syrian capital, Damascus, reports say. The al-Nusra Front, a jihadist militant group, said explosives-filled vehicles had been blown up outside the Air Force Intelligence (AFI) complex in Harasta. Activists and residents said there were huge explosions, followed by clashes involving machine-guns and mortars. AFI is seen by some as the elite agency of Syria's intelligence empire. Though smaller than Military Intelligence, in the past it has played a leading role in operations against Islamist opposition groups, as well as covert actions abroad, and has a reputation for brutality. Opposition and human rights activists say hundreds of opponents of President Bashar al-Assad have been detained and tortured by AFI agents across the country since the uprising began in March 2011. The agency is currently commanded by Maj Gen Jamil Hassan, a member of President Assad's minority Alawite sect. 'Notorious' A statement issued by the al-Nusra Front said it had targeted Air Force Intelligence "because it is one of the most notorious security divisions, and a citadel of repression whose extent is known only to God". The group, which has claimed to be behind a series of deadly bombings since January, said a vehicle packed with nine tonnes of explosives had been driven up to the AFI compound in Harasta and blown up. Twenty-five minutes later, another al-Nusra Front fighter blew up a one-tonne bomb hidden inside an ambulance, targeting survivors, it added. The fighters then shelled the area with mortars, the statement said. The Syrian Observatory for Human Rights, a UK-based activist group, reported that several large explosions had shaken homes overnight in Harasta, as well as in the nearby suburbs of Jobar and Qabun. The blasts were followed by violent clashes in several parts of Harasta, with government forces firing heavy weapons and mortars, it added. "We cannot confirm the exact number of casualties right now, but we expect the number is very high," the head of the SOHR, Rami Abdel Rahman, told the AFP news agency. A Syrian official told the Associated Press that a suicide bomber had detonated a car bomb near the AFI complex, while the pro-government TV channel, al-Ikhbariya, reported that the blast was followed by clashes.