Aleppo, Syria ? Two suicide car bombers struck security compounds in Aleppo on Friday, killing 28 people, Syrian officials said, bringing significant violence for the first time to an industrial center that has largely stood by President Bashar Assad during the 11-month uprising against his rule.

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 a popular uprising.

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

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. At the barracks, Brig. Firas Abbas told an Associated Press reporter on a government-guided visit to the scene that the vehicle made it through one roadblock before detonating near the gates. A head lay on the ground and security officials said it belonged to a suicide attacker.

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

State TV blamed “terrorists.” But anti-government activists accused the regime of setting off the blasts to discredit the opposition and avert protests that had been planned in the city on Friday.

Capt. 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 yards from the Directorate about an hour before the explosion but did not carry out the bombings.

“This explosion is the work of the regime to divert world attention from the crimes it is committing against the people of Homs,” he said.

The blasts were the fourth such dramatic suicide attack since late December. All occurred on Friday mornings against various security headquarters and prompted the same exchange of accusations.