NEW YORK — City officials yesterday increased security in New York's subways, citing what they called a credible terrorist threat against the nation's largest mass-transit system and its 4.5 million daily riders.

But authorities with the Department of Homeland Security in Washington, D.C., downplayed the threat, saying it was of doubtful credibility.

U.S. intelligence officials said U.S. forces who earlier in the week had raided a base belonging to a group linked to al-Qaida south of Baghdad, recovered information that 19 suicide bombers using suitcases and baby carriages planned to hit the New York subway system.

New York officials responded by mobilizing police officers to begin looking through commuters' strollers, bags, briefcases and luggage.

Other officials cautioned that the information that triggered the warning was shaky.

"This is the first time we have had a threat with this level of specificity," Mayor Michael Bloomberg said at a nationally televised news conference alongside Police Commissioner Raymond Kelly, adding that he still felt secure enough to take the subway home last night.

But in Washington, Homeland Security Department spokesman Russ Knocke said "the intelligence community has concluded this information to be of doubtful credibility. We shared this information early on with state and local authorities in New York." Knocke did not elaborate.

The national color-coded threat level remains at yellow, and New York remained at its higher level, orange.

A counterterrorism official, who was briefed about the threat by Homeland Security authorities, said the intelligence was considered doubtful because it did not reflect "on-the-ground, detailed" information.

Rather, the official, who also insisted on anonymity, said the intelligence was similar to "what can be found on the Internet and a map of New York City."

Bloomberg said no suspects had been arrested in New York or anywhere in the U.S.

New York officials knew about the threat for several days, but did not make any statements that could have "jeopardized the lives" of agents probing the plot, Bloomberg said. He added that one news organization had learned of the threat two days ago but did not make the news public at the request of government officials.

"Classified operations have partially disrupted this threat," Mark Mershon, assistant director of the FBI's New York office, said at the news conference. "The FBI and other U.S. personnel continue to work around the clock," he said, adding that the threat might be "resolved in the coming days."

Asked about possible links to the reported raid this week near Baghdad and the arrest of three people there, Mershon said, "I appreciate that there may be other reporting on this. Please understand our operations are classified, and we cannot discuss them."

FBI spokesman Richard Kolko had no comment on the differing NYPD and Homeland Security assessments of the potential threat, saying his agency "is aware of the information" and that it "has not been corroborated or discounted."

Compiled from Los Angeles Times, The Associated Press, Chicago Tribune and Knight Ridder Newspapers.