Suspected al Qaeda suicide bombers devastated a Riyadh residential compound of foreigners mostly from Arab states yesterday, killing between 20 and 30 people and injuring up to 100, diplomats said.

The huge blast which ripped an avenue of destruction between villas in the compound in the Saudi capital occurred after Western nations issued fresh terror alerts and Washington shut its missions in the kingdom, the world's biggest oil exporter.

The bombers shot their way into the guarded 200-villa Muhaya complex and detonated at least one car packed with explosives. Most residents were families of middle-class professionals from Egypt, Syria, Lebanon, Jordan and the Palestinian territories.

?This is a crime against innocents... It is an al Qaeda operation. This is a suicide operation,? a Saudi security source said. Saudi Arabia, birthplace of Islam, is battling a surge in Islamist violence blamed on al Qaeda.

Saudi authorities have yet to give official casualty figures, but the official Saudi News Press Agency said it would give details about the ?terror attack? later. Western diplomats and Saudi sources had conflicting tolls.

As rescuers searched amid rubble and raging fires, a senior Western diplomat said: ?We don't have an exact toll...but our best guess is that between 20 to 30 were killed and 50 to 100 were injured.? The Saudi source said earlier at least two had been killed and 65 injured.

One American was injured and another was reported missing, a US diplomat said, but it was unknown if they were of dual nationality. In Washington, a US State Department spokeswoman said: ?It appears that no US diplomats live at the compounds.?

Supporters of Osama bin Laden's al Qaeda have threatened Saudi rulers and Westerners who hold key jobs in the kingdom.

Royal homes nearby

A Western diplomat said Interior Minister Prince Nayef and other Saudi royals had private homes near the compound on the western outskirts of Riyadh. ?It was about half a mile from one of the houses of Prince Nayef?, the diplomat declared.

He said the compound might have been chosen as a ?soft target? after a recent tough crackdown by security forces. Security measures at compounds housing non-Westerners tended to be more relaxed.

A Reuters correspondent at the scene said: ?I saw two bodies, one being carried away and another lying on the grass of the compound. Whole walls of the 200 villas were blown out and glass covered the compound.?

The explosion gouged a crater five metres (yards) wide and two metres deep. Children's toys were strewn among the rubble and sofas, baths and beds spilled out onto a road dividing two rows of collapsed villas.

Soldiers, police, medics and firefighters rushed to try to find survivors under the debris, using detectors and search lights. Helicopters flew overhead and police sirens wailed. The acrid smell of explosives filled the air and smoke was rising from the rubble hours after the attack.

The powerful explosion shook windows in central Riyadh several kilometres (miles) away.

?I can see one of the (bombers') cars which is completely destroyed and I can see human remain?, said a Saudi television reporter. ?We don't know how many terrorists were in the cars.?

The attack, in the Muslim holy month of Ramadan, occurred six months after triple suicide bombings at Riyadh housing compounds on May 12 killed 35 people, including nine Americans.

Saudi Arabia blamed those bombings on al Qaeda. A purported bin Laden audio tape released last month promised more suicide attacks inside and outside the United States.

Heavy gunfire and one big explosion

The Saudi TV reporter said many of the injured taken away by ambulance were children under the age of 10. A Jordanian resident at the compound who identified himself only as Alaa said he heard heavy gunfire before the explosion.

?I heard shots, many shots, and then one big explosion. Many villas were damaged, four or five even collapsed. My house was far away but my windows were shattered? he said.

Arabic Al Arabiya television network showed footage of the injured covered in blood in hospital.

?Four villas out of a total of 200 are occupied by Westerners, the rest are Arab nationals?, Hanady al-Ghandakli, director of the Muhaya compound, told Al Arabiya. ?There is one French family, one British family, two German families, Italians?, she said. Arab television reports said three Americans and three Canadians of Arab origin were among the casualties.

?This is a crime against Arabs and Muslims and innocent people and those who carried out this evil act are defiling Islam?, one survivor told Al Arabiya.

Saudi forces have killed five Islamist militants in clashes since Monday, when authorities said they had foiled a planned attack on Muslim pilgrims in the holy city of Mecca.

On Friday, the United States issued its second security warning on Saudi Arabia in almost as many weeks, saying ?terrorists? were planning attacks in the kingdom. US missions were shut in the kingdom on Saturday for a security review.

After the Muhaya attack, the US embassy advised its personnel and dependants not to leave their compound.

Britain's embassies in the neighbouring Gulf states of Bahrain and Qatar warned British nationals of a high threat of terrorist attacks against Western targets in those countries.