The FBI has warned of possible terrorist attacks in New York, Los Angeles and Chicago around September 11.
Experts have told police that al Qaeda cells might use oil tankers and fuel trucks as weapons.
The vehicles would be stolen in order to inflict "mass casualties" on the fourth anniversary of the World Trade Centre attacks.
However, an FBI spokesman stressed the information was uncorroborated and the source of questionable reliability.
The Department of Homeland Security said there were no immediate plans to raise the national threat level.
Public transport systems in the US are still on high alert following the London bombings.
New York police commissioner Ray Kelly said the department was aware of the threat, Sky News reports.
"The New York City Police Department already has measures in place to protect against truck bombs and other threats," he said in a statement.
"We are expanding those measures, not in response to this latest information, but as part of ongoing refinements to our overall counter terrorism posture."
The alert was originally issued by the FBI in Los Angeles.
It warned: "Al Qaeda leaders plan to employ various types of fuel trucks as vehicle-borne improvised explosive devices in an effort to cause mass casualties in the US prior to the 19th of Sept."