The United States fears new attacks in Saudi Arabia
Alerts against possible terror attacks have been issued around the world, with warnings relating to a number of countries in Asia, Africa and the Gulf.
The warnings reflect concern that Osama Bin Laden's al-Qaeda network is planning more attacks on Western targets, following the triple suicide bombing in the Saudi capital Riyadh.
At least 25 people were killed in Monday night's blasts, along with nine attackers.
The US has now warned of a possible imminent attack in the western Saudi city of Jeddah.
"[We have] received an unconfirmed report that a possible terrorist attack in the Al Hamra district of Jeddah may occur in the near future," the State Department warning said.
Some diplomatic staff living in that district have already been moved to new quarters.
The US State Department has also advised Americans to defer non-essential travel to Kenya and to carefully review plans to visit East Africa in general.
The UK Government has abruptly suspended all commercial flights to and from Kenya.
Britons already there have been advised by the UK Government to keep a low profile and to be particularly vigilant in public places.
Foreign Office minister Mike O'Brien said a "growth" of intelligence had made the government "extremely worried".
He refused to be drawn on the specific nature of the threat, but said: "This is an area where there is al-Qaeda activity."
The Kenyan authorities have described the flight ban as a panic measure, saying it played into the hands of those who wanted to cause disruption. They insist that adequate steps have been taken to protect foreigners.
But they have also issued a photograph and details of a leading al-Qaeda suspect, Fazul Abdullah Mohammed, who is believed to have recently returned to Kenya.
The Bali nightclub bombings targeted Western tourists
He has been described as the chief architect of last November's bombing of a hotel near Mombasa which killed 15 people. He is also wanted by the FBI over the 1998 bombing of the US embassies in Kenya and Tanzania.
On the same day as the Mombasa bomb, there was an unsuccessful attempt to shoot down an Israeli charter plane as it left Mombasa.
Australia and New Zealand have meanwhile issued a warning to their citizens to be on alert in south-east Asia - a region still struggling to rebuild tourism following October's nightclub bombings on the Indonesian holiday island of Bali.
The Australian foreign office said Australians should be extremely cautious in Malaysia, Thailand, Singapore, the Philippines, East Timor and Brunei.
Mock terror drills
It follows an American warning of possible attacks in Malaysia, particularly in the state of Sabah.
The US has highlighted the threat allegedly posed by the Jemaah Islamiah, a militant South East Asian network believed to be linked to al-Qaeda.
It has also expressed concern about Abu Sayyaf guerrillas based in the southern Philippines.
In Lebanon, officials said the intelligence service - with help from Syrian forces - has arrested members of a group that was planning to attack the US embassy in Beirut.
Correspondents say Syria's reported involvement in the operation is significant because it comes at a time when both it and Lebanon are under pressure from the US to clamp down on radical groups.
In the US itself, emergency workers have been conducting a five-day drill dealing with mock bio-terrorism attacks in Chicago and a supposed radiological dirty bomb in Seattle.
The US administration has insisted that its war on terror has shattered al-Qaeda's leadership, but Homeland Security Chief Tom Ridge said: "The potential is still very, very real."