U.S. federal authorities have warned law enforcement officials in and around New York City of a possible al-Qaida terror plot against the city's transit system.

In a joint statement, the Department of Homeland Security (DHS) and the Federal Bureau of Investigation (FBI) said U.S. officials have received a "plausible but unsubstantiated" report indicating a threat during the upcoming holiday season.

The statement said the report indicates al-Qaida members may have discussed targeting New York during conversations in late September.  It said the plot could involve the use of suicide bombers or explosives placed on subway and passenger rail systems.

But the statement also said officials have no specific details to confirm the plot has developed beyond what was termed "aspirational planning."  Officials said they issued the warning as a precaution to law enforcement authorities.

New York City police spokesman, Paul Browne, says the department has deployed additional resources in the mass transit system in response to the warning.

U.S. federal authorities say they are not making any adjustment to the nation's threat level, despite the warning.

Some information for this report was provided by AP and Reuters.