The United States is in a heightened state readiness for a terrorist attack today after the Bush administration yesterday increased the alert status to orange, the second highest level of risk.
The US, Britain and Germany have ordered increased vigilance against terror attacks and have shut their embassies in Saudi Arabia in response to warnings that attacks may be imminent.
The warning follows suicide bombings that killed 75 people in Morocco and Saudi.
Saudi Arabia, where 34 people were killed by suicide bombers a week ago, said it had arrested three more suspected al Qaeda members.
In Washington Department of Homeland Security Undersecretary Mr Asa Hutchinson said the change in alert status was "based upon the recent terrorist bombings in Saudi Arabia and Morocco, also in conjunction with intelligence reports concerning anti-US terrorist groups' intentions."
US officials said there was no specific threat information but they would not rule out attempts to use "dirty bombs".
The FBI said earlier that the suicide bombings in Saudi Arabia and Morocco could herald an attack on the United States and a leading Saudi diplomat said he had a "gut feeling" that al Qaeda was planning a spectacular new attack.
The Irish embassy in Riyadh will remain open for the time being, the Department of Foreign Affairs said.
Washington, London and Berlin said they would shut their diplomatic missions in Saudi Arabia for the next few days.
Agencies