NEW YORK -- There are conflicting assessments about a possible security threat to New York's subway system.

New York officials increased security after saying they were alerted to a credible, but uncorroborated terrorist threat.

But officials from the Department of Homeland Security are downplaying the threat and say it is of "doubtful credibility."

Despite the differing takes on the seriousness of the threat, New York officials mobilized more police officers.

Mayor Michael Bloomberg said the threat originated overseas, and was the most specific terrorist threat New York officials had received to date. Bloomberg asked citizens to be vigilant. He also said he planned to ride the trains as usual.

Law enforcement sources said several terror suspects have been captured overseas, but there is concern that some members of a sleeper cell might be in the New York City area. Authorities have launched a manhunt to root out possible suspects.

The threat indicated the attack was planned for the second week of October -- perhaps around the time of elections in Iraq -- and specifically targeted trains in New York City, WNBC quoted sources as saying.

For the past week, the NYPD, federal law enforcement officials and international intelligence agencies have been trying to track the threat information. Because of the arrests overseas, they are considering the threat to be a credible one.

As a precaution, extra police will be on the trains and subways to increase security. The NYPD will immediately begin expanding its bag searches at subway stations across the city.