NEW YORK -- The FBI has warned US officials of a possible Al-Qaeda plot against New York's subway and transport grid, putting the city's police on heightened alert Wednesday.

Extra police were stationed around major hubs in the city even though the FBI said the threat, which came on the eve of the national Thanksgiving holiday, was unsubstantiated.

The FBI received "uncorroborated but plausible information that Al-Qaeda may have discussed a target at the transit system in New York City," Amy Kudwa, a spokeswoman for the Department of Homeland Security told AFP.

"Neither the Department Homeland Security nor the FBI has any specific information to confirm that this plot has developed beyond aspirational planning," she said.

The New York Police Department's deputy commissioner, Paul Browne, said authorities were "aware of an unsubstantiated report indicating that Al-Qaeda terrorists discussed targeting mass transit in New York City and vicinity."

He added that although there was no evidence to suggest the discussions had advanced any further, as a precaution "the NYPD has deployed additional resources in the mass transit system."

Extra police could be seen at Penn Station, a major subway and rail hub in Manhattan.

Six police cars were parked together outside as was a command truck at a separate location. Inside the station's hall for north-eastern Amtrak trains, five armed officers patrolled in helmets and flack-jackets, and with sniffer dogs.

"I see a lot of law enforcement, more than usual," said Wall Street worker Vishal Rawal, 25, who commutes daily between Manhattan and New Jersey.

"Of course I am concerned. Even if I am too busy with work, it is always on your back," he added.

The US terror alert level has not been raised, but riders on the New York subway were likely to see increased security, including uniformed officers and canine teams over the coming days.

"It is not uncommon for the department to receive threat information and to adjust our resources accordingly," Browne added.

On Thursday the Untied States marks the annual Thanksgiving holiday, when millions of Americans travel to see friends and family.

ABC television, citing law enforcement officials, said the plot involved a "Madrid-like attack" on Penn Station, a reference to the bombing of Spanish trains in March 2004 that killed 191 people.

Spanish courts last year ordered 21 people jailed for life in connection to the synchronized wave of bombs left on Madrid commuter trains on March 11, 2004. Four have since been acquitted after appeals.

Those attacks came less than three years after the September 11, 2001 attacks on the United States in which Al-Qaeda members riding hijacked planes slammed into the World Trade Center in New York, and into the Pentagon.

Just under 3,000 people were killed in those attacks.