Washington: Terrorists might try to target public transport systems such as trains and buses in major American cities this summer, using bombs hidden in bags or luggage, US counter-terrorism officials have warned.

A nationwide bulletin issued by the FBI (Federal Bureau of Investigation) and US Homeland Security Department late Thursday (Mar 25, 2004) on the basis of "uncorroborated" intelligence reports, said terrorists could try to conceal explosives in luggage and carry-on bags, possibly packing bombs made of ammonium nitrate and diesel fuel into such items as duffel bags and backpacks.

The security warning, coming within a month of the deadly March 11 commuter train attacks in Madrid in which bombs went off inside backpacks, did not mention any specific cities nor elaborated on the source of the information.

"Buses and railways could be targeted" it said, noting, "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 that the information in the bulletin was being shared via the National Law Enforcement Telecommunications System to ensure proper security measures are implemented around the country.

A spokesperson for the US passenger rail system Amtrak was quoted by media as saying that they stepped up security after the Madrid bombings, though there were no specific threats.