Two car bombs detonated outside security bases in the Syrian city of Aleppo on Friday, officials said, killing 28 people in the worst violence to hit the country's northern commercial hub in an 11-month-old revolt against President Bashar al-Assad.

State television showed blood, bodies and shattered concrete strewn across a street in front of a military intelligence building, and a crater several metres wide outside a security force base in the city.

Officials said the blasts were caused by explosives packed in two white minibuses, similar to those used by Syrian security forces. They said the vicitms included soldiers, civilians and children.

The explosions follow three suicide bombings in the capital Damascus in December and January which killed at least 70 people. Syria's government, blaming al Qaeda for at least some of those attacks, vowed an "iron-fist response".

Aleppo had been spared most of the unrest roiling Syria since anti-Assad protests erupted in March, but it has seen increasing protests and violence in recent weeks.

Reporters taken on a government-arranged visit to Aleppo saw a heavily armoured vehicle which had been tossed on its side by the force of one of the blasts. Windows in the five-storey intelligence building, about 50 metres (yards) from the site of the explosion, had been been blown out.

A reporter on state television, in a live broadcast from outside the complex shortly after the bombs shook Aleppo, said at least one blast had been audible 20 km (12 miles) away.

A concrete wall surrounding the buildings was badly damaged.

Lifting blankets and plastic sheets which had been laid over corpses on the pavement, the reporter showed a body with its head blown off and other bloodied human remains including a limbless torso and a blown off foot.

"We apologise for showing these pictures, but this is the terrorism which is targeting us," the reporter said, at times choking with emotion.

He said children were among the dead, showing a single rollerblade left on the pavement.

"Is this the freedom of Hamad and Erdogan?" one man shouted, referring to Qatari Prime Minister Sheikh Hamad bin Jassim and Turkey's Prime Minister Tayyip Erdogan, who have led the chorus of regional criticism against Assad. "Hamad, you dog," he said.

Syrian television showed similar bloody scenes at the second site, hit by what the reporter said was a car bomb. The blast had gouged a crater several metres (yards) wide, blown a truck onto its side and hurled chunks of concrete over a wide area.

The television showed people pulling body parts out of a black refuse bin for the camera.

The British-based Syrian Observatory for Human Rights said at least 30 people were killed in the explosions, adding that most of those killed were military personnel.

It also said seven people were killed in other districts oc Aleppo on Friday, but gave no further details.