The United States and Britain have warned new terrorist attacks in Saudi Arabia may be "imminent".
Countries have urged their nationals to remain vigilant
Both countries are temporarily closing their embassies in Riyadh and consulates in Jeddah from Wednesday, saying they have credible information about attacks planned against unspecified targets.
The announcements follow a warning on Monday from the Saudi ambassador to Washington, Prince Bandar bin Sultan, who said he feared a major attack was imminent in either the US or Saudi Arabia.
The US administration also decided on Tuesday to raise its terror alert status to "high", citing the renewed risk of attack.
The decision increases the alert level to orange - the second highest level on the administration's five-colour scale, officials said.
Prince Bandar said the US and Saudi Arabia had picked up electronic "chatter" in the region and in other areas, which indicated something else was being planned in addition to the suicide attacks that took place last week.
He said a raid by Saudi authorities earlier this month had uncovered huge quantities of explosives, which could have caused even greater devastation than the attacks which left 34 dead, including nine alleged bombers.
On Tuesday, Saudi Arabia said it was questioning three suspected members of al-Qaeda who were arrested in Jeddah on Monday.
The BBC's security correspondent Frank Gardner says three al-Qaeda cells were believed to have been active in Saudi Arabia prior to last week's attacks. One is said to have been destroyed, one fled abroad and the third remains active inside the country.
But the threat, he said, was not specific to Saudi Arabia - Egypt, Jordan, Yemen and Pakistan were also likely targets.
The UK Foreign Secretary, Jack Straw, said the closure of British missions would last for at least the next few days.
Germany also announced the closing of the consular section of its embassy and another mission in Saudi Arabia until the end of the week.
The US consulate in Dhahran as well as the UK trade office in al-Khobar are also being closed.
The US may not reopen its offices before Sunday, a message on the US embassy's website said. The UK Foreign Office said it planned to reopen its offices on Saturday but was keeping the situation under review.
A German foreign ministry spokesman said the German embassy in Riyadh and a mission in Jeddah would stay closed at least until Friday.
US State Department spokesman Richard Boucher said it was prudent to take "precautionary" measures as al-Qaeda remained a threat in Saudi Arabia.
The FBI also issued an advisory bulletin to all state and local police, saying its assessment was that attacks against US and Western targets overseas were likely. Attacks in the United States could not be ruled out.
"They [the attackers] were looking to do something more major than this," Saudi Prince Bandar told journalists on Monday.
He said chatter picked up by Saudi and US monitors had indicated the Riyadh attacks were being planned but had not given information regarding a time or place.
He said the chatter had stopped almost completely two or three days before the attacks.
"I'm confident we'll get them in the end. But the question is, will it be early enough or not?" he said.
Al-Qaeda has been blamed for the Saudi attacks
Saudi Arabia, which has been strongly criticised by Washington since the blasts, has pledged to do "whatever it takes" to uphold security.
Interior Minister Prince Nayef bin Abdul Aziz said the Saudi security forces were working closely with the US "in order to fight terrorism."
The suicide attacks came two weeks after the US announced it was withdrawing most of its troops from Saudi Arabia, where they were deployed during the 1991 Gulf War.