Most of the victims of last month's bombing were non-Saudi Arabs

It said at least one compound - the Seder Village - had been under "active surveillance by terrorist elements".

The warning comes three days after Britain urged its nationals to avoid non-essential travel to Saudi Arabia amid fears of fresh attacks.

Last month, a suicide bombing in Riyadh killed 18 people and injured 120.

Last week, Saudi officials said they had foiled a bomb attack when they discovered a car packed with more than a tonne of explosives.

Recent confirmed information indicates that the Seder Village housing compound in Riyadh has been under active surveillance by terrorist elements US Embassy in Riyadh

As the discovery was made, two suspected militants died in a shoot-out with police.

More than 50 people - including bombers - were killed in two suicide bomb attacks in the desert kingdom in the last six months.

Under growing pressure from the US, the Saudi authorities have been clamping down heavily on suspected militants since the first attacks on Western compounds in Riyadh last May.

Vigilance

"Recent confirmed information indicates that the Seder Village housing compound in Riyadh has been under active surveillance by terrorist elements," the embassy said in a statement.

"Other Western compounds within the kingdom may also be targeted," the statement added.

The embassy said it had already urged its employees and their dependants to avoid visiting Riyadh's housing compounds at night except for official meetings.

Embassy spokeswoman Carol Kalin told the Associated Press news agency that Americans living in the Seder Village had so far not been advised "to leave or evacuate".

But she said the embassy was renewing "its advice for Americans to remain vigilant and follow prudent security procedures".