Colour-coded alerts were issued after 11 September
It said that while it had no information about any specific plot, a general alert had been issued to banks in 12 states and the capital, Washington DC.
The warning came as the world's finance ministers gathered in Washington for a meeting of the World Bank and IMF.
The FBI did not reveal who was behind the threat.
Yellow alert
But the warning is thought to be based on information from captured members of the Al-Qaeda network, including one of its senior figures, Abu Zubaydah, who was arrested in Pakistan last month and turned over to the US.
Attorney General John Ashcroft stressed he was not asking banks to close branches or members of the public to stay away.
He said the government's policy was to issue warnings where appropriate in the hope that heightened vigilance would disrupt the plans of would-be attackers.
The FBI said the threat status for the north-east was unchanged at yellow, which represents the middle threat status.
None the less, there is a degree of nervousness in Washington, say correspondents.
Nuclear warning
The US developed colour-coded risk levels last month as part of a new "National Alert System" in the wake of the 11 September terror attacks that killed some 3,000 people in New York and Pennsylvania and outside Washington.
The warning is the latest in a series of alerts to be issued by the FBI and the newly established Homeland Security Council.
A general alert was issued in early December to cover the Christmas holiday period.
It was then extended to include the Salt Lake City Winter Olympics.
Other warnings to specific locations and industries - including nuclear power plants - have also been issued.