"The American embassy in Riyadh advises all American citizens living in Saudi Arabia that it has received indications of operational planning for a terrorist attack or attacks in the kingdom," the US embassy in Riyadh said in a statement.

"The embassy has no specific information concerning timing, target or method of any possible attack(s)."

Hours after the embassy published its warning, Saudi Arabia's Interior Ministry said security forces uncovered a hideout 60 km (40 miles) southeast of Riyadh where militants had stored more than two tonnes of bomb-making equipment.

The cache included fertiliser, ammonium nitrate, aluminium powder, potassium nitrate and other chemicals used by suspected al-Qaeda militants to make bombs deployed in attacks on Western housing compounds and security forces buildings.