WASHINGTON - FEDERAL authorities are warning law enforcement personnel of a possible terror plot against the New York City subway system during the holiday season.

An internal memo obtained by The Associated Press says the FBI has received a 'plausible but unsubstantiated' report that Al-Qaeda terrorists may have discussed in late September attacks against the subway system.

Department of Homeland Security spokesman Russ Knocke said the warning was issued as a routine matter, but added there may be an increased police presence in New York and other large metropolitan areas.

The report indicates that Al-Qaeda terrorists 'in late September may have discussed targeting transit systems in and around New York City. These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger rail systems,' according to the document.

'We have no specific details to confirm that this plot has developed beyond aspirational planning, but we are issuing this warning out of concern that such an attack could possibly be conducted during the forthcoming holiday season,' states the warning, which was dated Tuesday.

While federal agencies regularly issue all sorts of advisory warnings, the language of this one is particularly blunt.

Intelligence and homeland security officials were working with local authorities to try to corroborate the information 'and will continue to investigate every possible lead', the memo states.

Mr Knocke, the DHS spokesman, said the warning was issued 'out of an abundance of caution going into this holiday season'.

No changes are being made to the nation's threat level, or for transit systems at this time, he said.

'However, transit passengers in larger metropolitan areas like New York may see an increased security presence in the coming days.

This includes uniformed and plain-clothed behavior detection officers, federal air marshals, canine teams and security inspectors,' Mr Knocke said.

Thursday is the US Thanksgiving holiday, the unofficial beginning of the holiday season that ends with New Year's Day. -- AP