New York police stepped up security on Thursday after what city authorities called the most specific threat yet of a terrorist attack on the subway system in the coming days, but officials in Washington played down the threat.

Mayor Michael Bloomberg said the FBI had alerted him to "a specific threat to our subway system," which had come from overseas but had already been partially thwarted.

"We have never had before a threat to our subway system ... with this level of specificity," Bloomberg told a news conference. "We have done and will continue to do everything we can to protect this city."

In Washington, Homeland Security Department spokesman Russ Knocke said: "At this time the intelligence community believes that this information, while specific, is of doubtful credibility."

The information was shared with local officials "out of an abundance of caution" but there was no plan to raise the threat level for New York or the country, Knocke said.

Even so, New York police chief Ray Kelly said local authorities believed the information about an attack "in the coming days" was credible.

The city stepped up the number of uniformed and undercover police on America's largest transit system and Kelly said they would be on alert for people with baby strollers, briefcases and backpacks and they should expect to be searched.

Asked if the threat came from Islamic extremists, Bloomberg said, "I don't think we can characterize it coming from any ethnic group. It ... originated from overseas."

Still, Ana-Maria Matos, 28, rode the subway home to Brooklyn feeling nervous. "This makes it kind of scary to ride the subway. It's hard not to be biased against people, especially those of Middle Eastern descent," she said.

New York has been on high alert since the September 11, 2001 attacks on the World Trade Centre and bolstered security after the July 7 attack on London's transit system that killed 56 people. Those bombers carried explosives in backpacks.

During the evening rush hour, police were highly visible in New York's more than century-old subway system, which carries 4.5 million riders daily.

At Manhattan's Union Square, uniformed police conducted random searches, standing at tables with signs reading, "Backpacks and other containers subject to inspection."

Source: China Daily