U.S. authorities have warned of a possible al Qaeda threat to transit systems in and around New York City, a Homeland Security official said on Wednesday.

New York police said they were increasing security in response to the warning but said this was "in an abundance of caution." The warning comes at the start of the Thanksgiving holiday weekend, one of the busiest U.S. travel periods of the year.

"The New York City Police Department is aware of an unsubstantiated report indicating that al Qaeda terrorists discussed targeting mass transit in New York City and the vicinity," Deputy Police Commissioner Paul Browne told Reuters.

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Homeland Security Department spokesman Russ Knocke said the warning was issued on the basis of "plausible but uncorroborated information" and that al Qaeda may have discussed such attacks in late September.

Knocke said Homeland Security and the FBI had passed on the warning to state and local officials on Tuesday but there was no specific information to confirm that the plot had developed "beyond aspirational planning."

The warning was issued as a routine matter and no adjustments were being made to the nation's threat level.

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