The embassy district in Riyadh is surrounded by barricades

The Riyadh embassy shut just hours before a suicide bomb blast - blamed on al-Qaeda - killed 17 people and injured more than 100 in the city.

The US has boosted security at the embassy and its consulates in Jeddah and Dhahran.

"We do believe that it is now safe", said the US embassy spokeswoman.

Precautions urged

But US officials say they cannot rule out further attacks, and the Saudi authorities believe several al-Qaeda cells are still operating in the kingdom.

US and British officials in Saudi Arabia continue to warn their nationals to be vigilant.

American officials have urged US citizens in the kingdom to vary their times and routes for essential travel, park their cars in protected areas and check their vehicles before driving them away.

They have also advised Americans to avoid places and areas where Westerners are seen to congregate.

"Iron fist"

A Saudi-owned weekly news magazine based in London, al-Majalla, says it has received an e-mail from an al-Qaeda member claiming responsibility for the suicide bombing in Riyadh and threatening more attacks in the Gulf.

The Saudi authorities have said that anyone threatening security in the kingdom will be hit "with an iron fist".

However, the authorities denied reports that they had arrested a number of people in connection with the attack.