By Martin Asser BBC News

Security is usually tight around the US embassy

The Baathist government imposed a state of emergency when it came to power in 1963, giving the state security agencies the power to detain suspects for long periods without arrest warrants.

There are four major branches of security forces - the political security directorate, military intelligence, the general intelligence directorate and air force intelligence.

According to the latest US state department human rights report, these operate outside the control of the legal system and devote resources to monitoring internal dissent and individual citizens.

Observers say this situation has contributed to Syria's relative calm, compared with many of its neighbours which have been the target of a string of Islamist militant attacks over the years.

'Police state'

Violence such as that seen on a Damascus street on Tuesday outside the US embassy are therefore a rarity.

"It is rare because Syria is a very tight police state," former British ambassador in Syria Sir Andrew Green told the BBC.

"And the reason for that - which people often forget - is that Islamic extremists pose the main threat to the Syrian regime itself."

However, there have been isolated shooting incidents in Damascus's diplomatic quarter in recent months.

In June, four gunmen and a security guard were killed when Syrian security forces said they had foiled an attack by Islamist militants near the studios of state-run TV.

That is what led Britain's current envoy, Peter Ford, to speculate in media interviews that Tuesday's attack was the work of a small local group, rather than an attempt by the Islamist militant group al-Qaeda.

The timing of the attempted bombing could be significant though, 12 September, a day after the fifth anniversary of the 2001 attacks on the US carried out by 19 al-Qaeda suicide hijackers.

Foiled attack

The exact details surrounding the attack - which took place in one of the most heavily guarded areas of the Syrian capital - are still not clear.

Investigations will try to establish the background of the attack

However, television footage showed what looked to have been a powerful homemade bomb - pipe bombs strapped to large propane gas canisters - carried in a delivery van.

They did not get past the high perimetre fence of the US embassy, which is next to a public road. The embassy buildings themselves are some distance back from the fence.

Tuesday's violence appears to recall a 27 April 2004 incident in which the authorities said they had foiled an attack by Islamist militants in another diplomatic area.

Two men were sentenced to be hanged in connection with that attack. One of the men described his involvement as a "personal act" in a televised confession.

Syrian Interior Minister Bassa Abdel Majid said that while three attackers had been killed, one had been captured and it was hoped he would reveal the background of the attack.

Syria's security services can celebrate the fact that an attack on the US embassy was averted at a time of high tension between the two countries.

But questions will be asked about how the attackers managed to reach the embassy's first line of defence.