Intelligence showed al-Qaeda cells had been established in South Asia to carry out car bomb attacks, Attorney General John Ashcroft said.
Speaking on the eve of the 11 September anniversary, he also warned of suicide attacks in the Middle East.
The military has now been told to deploy live missiles in air defence systems at the Pentagon and around Washington.
The move came shortly after a heightened alert was declared in the US.
The BBC's Jon Leyne in Washington said Tuesday's statement by Mr Ashcroft was one of the most specific and detailed warnings issued by the US Government in a year of high alerts.
Precautions taken by the US authorities include:
'Alert but defiant'
Mr Ashcroft told a Washington news conference there was specific and credible information about threats to American symbols.
These included US military facilities, embassies and monuments.
Four embassies in South East Asia had been ordered closed.
"Information indicates that al-Qaeda cells have been established in several South Asian countries in order to conduct car bombs and other attacks on US facilities.
"These cells have been accumulating explosives since approximately January of this year in preparation for these attacks," Mr Ashcroft said.
"The US intelligence community has also received information that one or more individuals in the Middle East are preparing for a suicide attack or suicide attacks against US interests", he said.
But BBC security correspondent Frank Gardner says that if the threat does come from al-Qaeda it is out of character, because the organisation does not usually pay much attention to anniversaries.
The notion of striking US interests on the 11 September anniversary would make the organisation a little more predictable and perhaps a little easier to catch, our correspondent says.
There was no specific information at present about where the attacks might occur, Mr Ashcroft said.
Hand of al-Qaeda?
14 June 2002 Suicide bomb attack in Karachi targeting US consulate kills 11 people
11 April 2002 Bomb blast against synagogue in Tunisia kills 18
But he stressed that commemorative events in the US should go ahead as planned.
"We call on US people to remain alert but defiant in the face of this new threat", Mr Ashcroft said.
Tom Ridge, who heads the super-agency charged with overseeing domestic security in the wake of 11 September, added that the US was "at a high risk of a terrorist attack".
US Defence Secretary Donald Rumsfeld has now authorised the deployment of live surface-to-air missiles around Washington.
Fighter aircraft armed with live weapons are already patrolling the skies.