NEW YORK (CNN) -- New York City's subway system was put under heightened alert Thursday after officials received information from the FBI about a "specific threat," Mayor Michael Bloomberg said.

A well-placed U.S. military official told CNN on Thursday that the same intelligence also led to a raid against suspected al Qaeda operatives in Iraq.

There were indications that a terrorist attack on New York's subway system is possible "in the coming days," said New York Police Commissioner Raymond Kelly.

Both he and Bloomberg declined to give further details, citing security concerns. (Watch how the NYPD is ready for a threat -- 3:09)

Spokesmen for New Jersey transit systems and Amtrak said they remained on a heightened state of alert, as they have been since early July.

Law enforcement sources told CNN the threat information came from Iraq and that it included claims that a group of 15 to 20 people were in the United States to carry out an attack against New York's mass transit system.

The sources said they could not corroborate the information.

No arrests have been made in Manhattan, but operations continue outside the city, Kelly said, without mentioning where.

And Bloomberg said that officials "do not have any reason to believe" that any of the plotters "are in New York at this point in time"

"The encouraging news is that classified operations have, in fact, partially disrupted this threat," said Mark Mershon, assistant director of the FBI's New York field office.

It was the first specific threat against the city's subway system, Bloomberg said. But the threat did not mention specific subway stations, he said. (Watch New York's police commissioner explain the threat -- 3:02)

New York has 26 subway lines, 490 stations and 660 miles of subway track. There are about 6,400 cars.

"The detail of this specific threat is, in fact, classified," Mershon said. "We put down threats, multiple threats, every day. But the detail of this specific threat was so on point that we did raise this concern with the New York City Police Department."

Officials at different levels of government questioned the credibility of the information, however.

A Bush administration official told CNN the threat involved the use of explosives hidden in baby carriages. The official said he believes New York officials made the threat public "out of an abundance of caution."

An official from the Department of Homeland Security told CNN that the agency has received intelligence regarding "a specific but not credible" threat to New York's subway system "in recent days."

The official said that the intelligence community concluded the information is of "doubtful credibility."

Still, Bloomberg urged New Yorkers to be vigilant.

"If you see something, say something. Call 311 or 911, if it's an emergency," he said. (311 is New York City's phone number for government information.)

Bloomberg said he still plans to travel on the subway Thursday evening and Friday. "I have no problems with my daughters taking the subway any more than me," he said.

An average 4.5 million commuters use New York's subway system daily.

Kelly said that briefcases, baby strollers, luggage and other containers will draw particular attention, and he asked the public to curtail their use if possible.

In addition to more bag searches, the city has significantly increased police presence -- uniformed and undercover -- on the subways and at station entrances, Kelly and Bloomberg said.

In addition to extended shifts among police officers, hundreds of National Guard members and state and Metropolitan Transportation Authority police would help respond to the threat, New York Gov. George Pataki said in a statement.

New York has been on "orange" alert, or the second-highest level -- indicating a high risk of terror attack -- since the color-coded warning system was established after the September 11, 2001 attacks.
Source: Iraq connection

The highly classified operation involving the U.S. intelligence community and the U.S. military was carried out against suspected al Qaeda operatives south of Baghdad, CNN has learned from a well-placed U.S. military official.

"Significant intelligence was also gathered in this raid, along with al Qaeda operatives who were rounded up," CNN's Jamie McIntyre reported, citing the official who spoke on condition of anonymity.

Few other details were provided about the raid, which occurred in the town of Musayyib, in the lawless area nicknamed "The Triangle of Death." But the official said the raid was the result of the same intelligence about the threat to New York's subway system.

The news came 10 days after the arrest of nine people in France allegedly plotting attacks on the Paris subway, an airport and an intelligence agency's headquarters. (Full story)

"We can't be too careful, and the precautions that New York City has taken are appropriate and make a good deal of sense," said Sen. Charles Schumer of New York, a Democrat. "We cannot be in a situation where we don't turn over every stone. But the bottom line is that in the post-911 world, you can't be too careful."

In July, London's public transportation system was attacked twice. At least 52 people died and 700 people were wounded when four men detonated bombs aboard subway trains and a double-decker bus on July 7. The morning rush-hour attacks made it the city's bloodiest day since World War II.

Nobody was killed when simultaneous attacks were carried out on three London subway trains and a bus two weeks later, on July 21.

In a speech Thursday, President Bush said that since September 11, 2001, the United States and its allies have thwarted 10 al Qaeda attacks worldwide, three of them inside the United States. (Full story)

Source: CNN