New York City and federal officials said Thursday evening that they had received information about a specific but unconfirmed terrorist threat to the city's subway and vowed to tighten security in transit system of the city as a precaution.

"This is the first time we have had a threat with this level of specificity," New York Mayor Michael Bloomberg told a press conference that was carried live by the local news broadcasts.

But he said the city would not raise its color-coded threat level from "orange," the second-highest in the system used since the Sept. 11 terror attacks, and he urged New Yorkers to continue using the subway system, as he said he would.

"I believe people should live their lives as they always do and have faith in the world's greatest police department," the mayor said.

Police Commissioner Raymond Kelly, however, suggested that riders not bring backpacks, briefcases, baby carriages and luggage if they can do without them. Police were stepping up random bag searches that began after London bombings this summer.

Media reports, citing some unidentified security sources, said a US secret service operation in Iraq had taken two people there into custody to thwart a plot that involved a dozen or so people who attempted to plant explosives in the subway system around the middle of this month.

But officials present at the news conference, would only allude to an overseas operation and declined to provide details about detentions, saying only that they had taken place outside New York.

"Classified operations have partially disrupted this threat," Mark Mershon, assistant director of the Federal Bureau of Investigation's New York office, said at the news conference.

"The FBI and other US personnel continue to work around the clock. Nothing that has surfaced in that investigation has corroborated an actual threat to the city."

Mayor Bloomberg said the city has known about the threat for several days, but decided that making it public too soon "could have jeopardized the lives" of some investigators.

"We have done and will continue to do everything we can to protect this city. We will spare no resource, and we will spare no expense. We have increased our police presence on our subways," he said.

The mayor urged New Yorkers to remain vigilant and report any unattended packages, bags, or suspicious activities.

Source: Xinhua