Two booby-trapped cars blew up at security sites in Damascus on Friday, killing a number of civilians and soldiers, state television said, in the worst violence to hit Syria's capital during nine months of unrest against President Bashar al-Assad.

Al Manar, a television news channel owned by Lebanese Shi'ite Muslim militant movement Hezbollah, a major ally of Assad, put the number of dead from the blasts at 30 with 55 wounded and said most of the casualties were civilians. Al Manar cited information from its own correspondents at the scene.

Syrian television described the attack as a suicide bombing and said initial inquiries indicated al Qaeda was behind it.

The attack came a day after the arrival of Arab League officials to prepare for a monitoring team that will check whether Assad is implementing a plan to end the bloodshed.

State television broadcast footage of bloodied bodies being carried in blankets and stretchers into ambulances and people hunting through rubble of a badly damaged building.

A Reuters cameraman was barred from the site. State television also broadcast shots of bloodied streets littered with mangled human remains and blackened debris.

State television said the blasts targeted a state security administration building and a local security branch.

The United Nations says Assad's forces have killed more than 5,000 people in their crackdown on the protests, which erupted in March inspired by uprisings in Tunisia, Egypt and Libya.

Syria says it faces a campaign by foreign-backed gunmen and terrorist groups. This week it said more than 2,000 members of the army and security forces had been killed since March.

Anti-Assad protests have swept the country, although central Damascus and the northern commercial city of Aleppo have remained relatively quiet.

A small blast was reported near a Syrian intelligence building in Damascus last month, but there was little damage.

But in recent months the mainly peaceful pro-democracy movement has become overshadowed by pockets of armed insurgency that have launched attacks on Syrian security forces.

The escalating violence on both sides has raised fears that the country is slipping towards civil war.

ARAB MONITORS

An advance team has arrived in Syria to prepare the way for Arab League monitors who will judge whether Damascus is honouring a plan it agreed last month to stop the violence.

The peace plan calls for a withdrawal of troops from the streets of protest-hit cities and towns and their surroundings, release of prisoners and dialogue with the opposition.

Arab League sources have said the advance team, led by top League official Samir Seif al-Yazal, comprises a dozen people, including financial, administrative and legal experts tasked with ensuring monitors can move freely across Syria.

The main group of around 150 observers is to arrive by the end of December. Syria stalled for six weeks before signing a protocol on Monday to admit the monitors.

Activists say Assad, 46, is still trying to stamp out protests with troops and tanks despite international sanctions and his avowed agreement to the Arab League plan.

The Syrian Observatory for Human Rights said Assad's forces carried out major assaults in the northern and southern provinces this week, apparently trying to crush opposition to strengthen his hand before the arrival of the monitors.

It said troops surrounded and killed 111 people on Tuesday in the northern province of Idlib, in the deadliest assault since the uprising erupted.

France called Tuesday's killings in Idlib an unprecedented massacre. The United States said Syrian authorities had flagrantly violated their commitment to end violence while Assad's former ally Turkey condemned Syria's policy of oppression which has turned the country into a bloodbath.

Syrian officials say more than 1,000 prisoners have been freed since the Arab plan was agreed and the army has pulled out of cities. The government has promised a parliamentary election early next year as well as constitutional reform that might loosen the Baath Party's 48-year grip on power.

Syrian pro-democracy activists are deeply sceptical about Assad's commitment to the plan. If implemented, it could embolden demonstrators demanding an end to his 11-year rule, which followed three decades of domination by his father.

The British-based Avaaz rights group said on Thursday it had evidence of more than 6,237 deaths of civilians and security forces in the conflict, 617 of them under torture. At least 400 of the dead were children, it added.

(Writing by Dominic Evans)