US authorities fear car or truck bombs may be used against Wall Street firms or financial institutions in Washington. Some media outlets said potential attacks might involve suicide truck bombs.
US authorities on Sunday increased the terrorist alert threat level in Washington to "high" following a new warning by New York police of a possible al Qaeda attack, CNN television reported.
There was no immediate confirmation by the Homeland Security Department.
New York has been on a "high" alert ever since it was the main target of the September 11, 2001 attacks when airliners hijacked by al Qaeda members slammed into the World Trade Center towers leaving almost 3,000 dead.
The New York Police Department (NYPD) said in a statement released late Saturday that "intelligence reporting indicates that al Qaeda continues to target for attack commercial and financial institutions, as well as international organisations, inside the United States."
The statement called on managers of buildings to be alert for unannounced maintenance workers and deliveries.
Police warned building managers and corporate security personnel to watch for vehicles that could be rigged with explosives and for chemical agents placed in ventilation systems.
Media reports said US intelligence services considered the new threat to be "credible". The reports said police and security officials had spent the weekend meeting representatives of major firms in New York and that police SWAT tactical teams had been sent out across the city.
There were no evident signs of increased security however.
The report gave no indication on the timing of any attacks on the cities that were also the target of the devastating September 11, 2001 assaults by al Qaeda.