"We assess that buses and railways could be targeted," the FBI and the Department of Homeland Security said in a bulletin issued to law enforcement agencies, local governments and the transportation industry.

"The plot calls for the use of improvised explosive devices possibly constructed of ammonium nitrate (fertilizer) and diesel fuel concealed in luggage and carry-on bags to include duffel bags and backpacks," according to the bulletin.

It said, "al-Qaeda and other groups have demonstrated the intent and capability to attack public transportation with conventional explosives, vehicle-borne bombs and suicide bombers."

Homeland Security spokesman Brian Roehrkasse said the bulletin was issued in light of recent events, such as the March 11 train bombings in Madrid, which killed 191 and injured 1,500.

He said officials believe the plot could target mass transit by using bombs concealed in bags, as they were in Madrid.

"In the past, bus bombings have been carried out in Greece, Israel and Turkey," according to the bulletin.

"Terrorist manuals provide instruction on producing a number of ammonium nitrate explosive mixtures" and such a bomb could be concealed in standard luggage, it stated.

The bulletin said anyone with any information about the possible bombing plot should contact the nearest FBI joint terrorism task force.

Homeland Security undersecretary Asa Hutchinson said the department had been in regular contact with the transit sector to share intelligence and make recommendations to boost security.

"So there is an increased law enforcement presence, there is increased public announcements (and) other security measures are being put in place," Hutchinson told reporters.

"Action has intensified in a common-sense way in response to what we have seen from intelligence and actions overseas," he said.

US Senator Hillary Rodham Clinton urged spending more than $700 million to overcome security weaknesses in the US rail system.

"What happened in Madrid was a big wake-up call for a lot of people who may not have been focused on the need to improve rail security," Clinton, a New York Democrat, told reporters at New York's Pennsylv-ania Station.

Earlier on Friday in Spain, a bomb was found on a high-speed track. Spanish authorities said it may have contained the same type of explosives used in last month's Madrid train bombings.

The Department of Homeland Security said terminal parking lots should be checked for suspicious loitering, stolen or abandoned vehicles or vehicles left for long periods. Law enforcement vehicles should be parked near entrances and exits.

The agency said trash receptacles should be removed from passenger terminals, and recommended random passenger inspectors and security sweeps of stations.

The department also urged increased public safety announcements encouraging passengers to immediately report all suspicious activity or items to law enforcement authorities.

"Safeguard uniforms, patches, badges, ID cards and other forms of official identification as terrorists may exploit such items to gain easier access to public areas or other areas not normally accessible by the general public," it warned.