Anti-Assad activists denied any involvement and accused the regime of setting off the blasts to smear the opposition as government forces pummel rebels in one of their main strongholds, Homs. State media touted the bombings as proof the regime faces a campaign by terrorists, not an uprising.

The military stepped up its siege of Homs, where hundreds have reportedly been killed during the past week. Soldiers who have been bombarding the central city made their first ground move, storming into one of the most restive neighbourhoods.

Troops and security forces opened fire on anti-regime protesters, who streamed out of mosques after Friday prayers nationwide. At least 27 civilians were killed, according to the London-based Syrian Observatory for Human Rights.

The morning blasts in Aleppo, Syria's most populous city, ripped apart the facades of the local headquarters of the Military Intelligence Directorate and a barracks of the Security Preservation forces.

At the Directorate, windows were shattered and a large crater was torn into the pavement outside the entrance. A weeping correspondent on state-run TV showed graphic footage of at least five corpses, collected in sacks and under blankets by the side of the road.

Security officials said suicide bombers in explosives-packed vehicles tried to smash through the entrances of both sites.

State television cited the Health Ministry as saying 28 people were killed in the two blasts and 235 were wounded. The dead included 11 security personnel at the barracks, 13 military personnel at the military intelligence building and four civilian.

Captain Ammar al-Wawi of the Free Syrian Army, a rebel group that wants to bring down the regime by force, denied involvement. He said fighters from his group had a short gunbattle with troops several hundred metres from the Directorate about an hour before the explosion but did not carry out the bombings.