THREAT IN US: The deadly train bombings in Spain have had an immediate effect in New York, where security has been stepped up on subway and metropolitan rail lines.
In Washington, US President Bush attended a wreath-laying ceremony at the home of the Spanish Ambassador, Mr Javier Ruperez, to express solidarity with the people of Spain, an ally in the war on Iraq.
Mr Bush called the bombings "a grim reminder there are evil people in the world willing to kill innocent life". He said he appreciated very much the Spanish government's "strong stand against terrorists and terrorist organisations like Eta".
In all big US cities, but particularly in New York, the carnage in Madrid brought home to commuters and security officials alike just how open their transit systems are to a similar attack.
"The bottom line is, we are vulnerable," said New York Democratic Senator, Chuck Schumer, who called the two-kilometre Hudson River tunnel leading out of Penn Station a particular "death trap".
Mayor Michael Bloomberg called the attacks a warning of the deadly potential of terrorists if they decided to strike New York again. "It goes to show we still live in a very dangerous world," he said. "We cannot afford to let our guard down."
Most US newspapers carried prominently the claim of responsibility by an al-Qaeda-linked group for the bombings in Spain, and its warning that a "winds of black death" strike against America was "90 per cent ready" and "God willing, near".
The White House has treated the claim with some scepticism as the same group also claimed responsibility for last year's electricity black-out in the north-eastern United States.
But with no definite conclusion on who carried out the Madrid bombings, plainclothes officers who patrol NY subways were changed into uniforms and patrols on the platforms, stations and in the trains increased.
National Guard troops called up by Governor George Pataki watched the entrances to the main stations and extra police from the Metropolitan Transportation Authority were deployed on the Long Island railroad lines.
Security has been tightened up considerably since the 9/11 attacks, and picture ID is required to buy tickets on the Amtrak inter-city services, but luggage is never inspected and experts say that it is impossible to screen the millions of passengers who use the subway daily. New York is on a higher level of alert than the rest of the US as it is considered a prime al-Qaeda target.