Suicide car bombers struck Damascus on Friday, officials said, killing 40 people, gutting buildings and sending human limbs flying in the bloodiest violence seen in Syria's capital since a revolt against President Bashar al-Assad began nine months ago.

The attacks, which state media blamed on al Qaeda, targeted two security compounds and 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.

Assad has unleashed tanks and troops to try to crush a wave of street protest inspired by other Arab uprisings this year. Mainly peaceful rallies are now increasingly eclipsed by an armed insurgency against his military and security apparatus.

But Friday's blasts in central Damascus signalled a dramatic escalation in violence which Syrian authorities blame on armed groups they say have killed 2,000 soldiers and security force members this year. The United Nations says Assad's crackdown has killed 5,000 people.

It's a new phase. We're getting militarised here, said Joshua Landis, a Syria expert at the University of Oklahoma who felt Friday's bombs were a small premonition of what may come in a country that some analysts see slipping towards civil war.

This is when the Syrian opposition is beginning to realise they are on their own, he added, referring to Western reluctance to intervene militarily in Syria. We're getting into a cycle of revenge. An eye for an eye.

State television said over 150 people were wounded by the explosions. It broadcast footage of mangled bodies being carried in blankets and stretchers into ambulances with sirens wailing.

The footage also showed bloodied streets littered with human remains, blackened hulks of cars and other debris, as well as a row of corpses wrapped in sheets laid along a street.

The television said the bombers zeroed in on a state security administration building and a local security branch.

Syria has generally barred foreign media from the country, making it hard to verify accounts of events from either side.

Foreign Ministry spokesman Jihad Makdesi said the attacks were carried out by terrorists (trying) to sabotage the will for change in Syria, and followed warnings from Lebanon that al Qaeda fighters had infiltrated Syria from Lebanese territory.

There was no immediate claim of responsibility. Al Qaeda are Sunni Muslim militants. Assad and Syria's power elite belong to the Alawite branch of Shi'ite Islam while the majority of Syrians, including protesters and insurgents, are Sunnis.

Assad's opponents said the attack could have been staged to drive home the government's argument. We have all sorts of suspicions that this could be organised by the regime itself, said Basma Qadmani, spokeswoman for the Syrian National Council.

WAR IN SYRIA

But analysts said authorities were unlikely to have staged an attack that only serves to highlight their vulnerability.

There is a war going on in Syria now, said Rami Khouri, director of the Issam Fares Institute in Beirut, saying various armed rebel or Islamist groups could have staged the bombings.

The last serious attack of its kind in the tightly controlled Syrian capital was three years ago when a car bomb went off near a security complex, killing 17 civilians.

That attack, for which Syrian authorities blamed an Islamist suicide bomber using a car brought in from a unnamed neighbouring Arab country the day before, was one of the biggest in Damascus since an Islamist militant campaign in the 1980s against Hafez al-Assad, late father of the current president.

Hilal Khashan, a political scientist at the American University in Beirut, said neither the government nor al Qaeda were likely to have been responsible for Friday's attack.

When it comes to security in Damascus...the government does not play games, he said. I think this is the symptom of desperation after so many Syrians have seen blood and death in the crushing of protests.

The United Nations says Assad's forces have killed more than 5,000 people in their crackdown on the protests, which erupted in March instigated by uprisings that toppled autocratic leaders in Tunisia, Egypt and Libya over the course of the year.

The intensifying violence on both sides in Syria has raised fears that the country is drifting towards civil war. Downtown Damascus and Syria's second city Aleppo hitherto had largely escaped the turmoil now common in many other cities and towns.

First I heard an explosion and then, all of a sudden, I saw human limbs flying everywhere, said one man interviewed by Syrian television near the site of the attack in Kafr Sousa.

His head and face were covered in bandages.

State media said the Arab League delegation, which will be seeking assurances of free movement for 150 monitors due to arrive in Syria by the end of the month, had visited the sites of the explosions to inspect the damage.

The monitors are supposed to verify Syria's implementation of an Arab League peace plan it agreed six weeks ago, which stipulates a withdrawal of tanks and troops from the streets of protest-hit cities and towns and their surroundings, release of prisoners and reform-minded 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.

Activists say Assad, 46, is still trying to suppress public dissent with military force despite being hit with European Union and Arab League sanctions, and his avowed commitment to the peace 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 quell serious opposition to strengthen his hand before the monitors settle in.

Syrian troops surrounded and killed 111 people on Tuesday in the northern province of Idlib, in the deadliest single assault since the uprising erupted, according to the Observatory.

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.

Damascus says more than 1,000 prisoners have been freed since the Arab plan was agreed and the army has pulled out of cities. Anti-Assad activists deny any such pullout has occurred.

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 said.

(Additional reporting and writing by Dominic Evans)