An attack by army defectors on a major intelligence complex in a Damascus suburb left 20 security police dead or wounded, prompting retaliation by security forces on the neighbourhood, residents and opposition sources said on Thursday.

Wednesday's attack on an Air Force Intelligence complex in Harasta was the first on a major security target in the eight month uprising against President Bashar al-Assad, indicating that fighting could be spreading to Syria's centres of power.

The defectors used rocket-propelled grenades and machineguns and managed to inflict casualties on those who were within the ... outer wall, said an operative involved in supplying the defectors.

There was no confirmation of the attack from authorities. Syria's ban on most foreign media makes it hard to verify reports of events on the ground from activists or officials.

One of the residents of the suburb, who declined to be named, said there were no casualties among the attackers, and that the operation lasted for 10 minutes. He said the defectors were mostly from the Damascus suburbs of Harasta and Douma, which have taken the brunt of a crackdown on protests demanding Assad's removal in rural Damascus.

Residents said around 70 people have been arrested in the last 24 hours, with Air Force Intelligence agents raiding houses and destroying several businesses.

Roadblocks have been set up everywhere in Harasta, especially in al-Seil neighbourhood, where activists are concentrated. Five textile workshops were ransacked, another resident said.

Air Force intelligence trucks are patrolling the suburb also, and the agents are carrying RPGs, he said.

Together with Military Intelligence, Air Force Intelligence is in charge of preventing dissent within the army. The two divisions have been instrumental in a crackdown on the uprising against Assad which the United Nations says has killed 3,500 people.

Syria blames the violence on armed groups backed by foreign powers, and says more than 1,100 soldiers and police have been killed since the uprising erupted in March.

Syria's military is controlled by Assad's brother Maher and members of their minority Alawite sect, while the army is comprised mostly of Sunni Muslims, who also form the majority of Syria's population and have been defecting from the army in mounting numbers.

The pervasive security apparatus, dominated by Alawites, underpins the power structure. Security chiefs of an estimated eight major secret police organisations answer directly to Assad.

(Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Jon Hemming)