Military facilities across the United States have been placed on the highest state of alert following renewed threats of terror attacks.
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The move comes after defence intelligence experts raised their assessment of the risk of attack.
The US as a whole remains on the second-highest state of alert - orange in the US colour-coded scale.
With Americans preparing for the Memorial Day long weekend, the authorities are urging citizens to be extra vigilant but not to change their plans.
Security has been boosted at what are considered to be high-profile targets in major cities.
There are also fears of terrorist attacks against US and other Western offices in Saudi Arabia, following last week's suicide bombings in Riyadh.
The US, Britain, Germany and Italy have temporarily closed their embassies and consulates in the kingdom.
'Warnings to Muslims'
National Guardsmen have been deployed to protect New York's subway system and security has also been boosted at the Sears Tower in Chicago and the Golden Gate Bridge in San Francisco.
Al-Qaeda has been blamed for the Saudi attacks
"The US intelligence community believes that al-Qaeda has entered an operational period worldwide, and this may include attacks in the United States," Homeland Security Secretary Tom Ridge said.
Possible terror tactics, he said, included "small-arm-equipped assault teams, large vehicle-borne explosive devices and suicide bombers".
The ABC television network has been quoting two e-mails intercepted by US intelligence which it said it had obtained from an FBI bulletin.
One warns of a "possible devastating attack in the next 48 hours" and urges Muslims to leave all cities, especially Boston and New York.
The other mentions Washington DC and New York as places for Muslims to avoid, as well as America's beaches.
The BBC's Justin Webb says the last time the US was put on the orange state of alert was during the war in Iraq.
The authorities now believe al-Qaeda may target the US to demonstrate it is still viable, he adds.
Mr Ridge has also encouraged state governors and city mayors to deploy extra police across the nation.
Our correspondent says that most will comply, but the costs are high and complaints are growing that without specific intelligence the alert system causes alarm without doing much practical good.
The US consulate in Dhahran as well as the UK trade office in al-Khobar are also being closed to the public.
The US may not reopen its offices before Sunday, a message on the US Embassy's website said.
The UK Foreign Secretary, Jack Straw, said the closure of British missions would last for at least the next few days.
A German foreign ministry spokesman said the German embassy in Riyadh and a mission in Jeddah would remain closed at least until Friday. The Italian embassy also closed its doors to the public on Wednesday.
The suicide attacks on Riyadh, which left 34 people dead, came two weeks after the US announced it was withdrawing most of its troops from Saudi Arabia, where they have been deployed since the 1991 Gulf War.