New York Mayor Michael R. Bloomberg warned residents Thursday night of "a specific threat" of a terrorist attack on the city's transit system in coming days, but Washington authorities played down the threat as "non-credible."

Bloomberg (R) said he stepped up security for the transit system and made the announcement because "we have an obligation to share information with the public, as long as it doesn't jeopardize their safety, and they can make their own decisions."

The mayor assured residents that everything was being done to provide security, and he vowed to ride the subway to work as usual Friday. He said the threat "was more specific to target" and "more specific to timing" than previous threats, came from multiple sources, and was the most detailed ever received against the 4.7 million-rider-a-day system.

Appearing with Bloomberg, the FBI's assistant director for New York, Mark J. Mershon, said the U.S. government and allied law enforcement agencies "partially disrupted" a plot.

But a Bush administration official, who spoke on the condition of anonymity because details of the investigation are classified, said "recent operations abroad" determined that the threats were dubious.

In a statement, the Department of Homeland Security said, "The intelligence community has concluded that this information is of doubtful credibility."

Department spokesman Russ Knocke added, "Out of an abundance of caution, DHS shared this information in recent days with state and local authorities in New York," and will continue to review and analyze data "in full coordination" with them and with intelligence community members.

He said there are no plans to raise the threat levels for New York (which is at orange, or "high") or the country in general (yellow, or "elevated").

CNN reported that U.S. military and other government agencies conducted a raid south of Baghdad on Wednesday night based on the same threat information passed to New York officials, rounding up al Qaeda operatives and intelligence. New York police said they made the announcement after two al Qaeda operatives were arrested in Iraq. Spokesmen for the Pentagon in Baghdad and Washington said they were looking into reports of a raid and any connection to New York.

In a 6 p.m. briefing, Bloomberg said the FBI notified the city of the threat in recent days and NBC News was asked Tuesday to withhold reporting to permit the investigation to proceed.

The Port Authority of New York and New Jersey, the Metropolitan Transit Authority, and Amtrak were coordinating security measures for buses and ferries. More uniformed and undercover police officers were put at subway stations, trains will undergo more maintenance sweeps, and passenger briefcases, baby strollers, luggage and other containers will be subject to added scrutiny, said New York Police Commissioner Raymond W. Kelly.

Hsu reported from Washington. Staff writers Lyndsey Layton and Ann Scott Tyson and researcher Julie Tate contributed to this report from Washington.

A train stops at Times Square during the evening rush hour, shortly after the warning.