Western interests have been previously targeted in Saudi Arabia

The move came as Washington warned that terrorists might be planning to hijack cargo aircraft overseas and crash them into targets in America.

In particular, officials responsible for safety at nuclear plants, bridges and dams have been made aware of the potential threat.

But the authorities admit the suggestion has come from a single source and has so far not been corroborated.

Terrorist funding

The US Secretary of State Colin Powell in an interview with a London-based Saudi newspaper Ash-Sharq Al-Awsat has said he thinks the Saudi authorities could do more to combat terror.

Saudi Arabia could make greater efforts in pursuing the sources of terrorist funding, he told the newspaper.

State Department spokesman Richard Boucher on Friday said fears of an imminent attack in the kingdom meant it was "prudent... to warn Americans and to close our operations for a review."

The US embassy in Riyadh, as well as the missions in Jeddah and Dhahran, were closed on Saturday.

"We will take it day by day," said a US embassy official when asked when the operations would re-open.

US sites in Saudi Arabia have previously been targeted by attackers.

Call for vigilance

The US embassy in Riyadh said it "continues to receive credible information that terrorists in Saudi Arabia have moved from the planning to operational phase of planned attacks in the kingdom."

The statement said: "The embassy strongly urges all American citizens in the kingdom to be especially vigilant when in any area that is perceived to be American or Western."

Britain, Canada and Australia last month issued similar alerts which angered Saudi officials, who say they have made important strides in fighting terror inside the country.

Spate of trouble

Saudi police in Mecca recently uncovered a suspected al-Qaeda cell believed to be planning attacks.

Saudis grapple with change

On Thursday, two suspected members of the cell blew themselves up apparently to avoid arrest, while a third suspect was shot dead by security forces in Riyadh.

In May 35 people were killed in suicide attacks on a Western compound in Riyadh.

"We're very much on guard, very much aware of what's going on. I'm extremely wary," David Castillo, vice-president of a telecoms company in Saudi Arabia told Reuters news agency.