Federal officials have warned authorities in New York City, Texas and Virginia about an unspecific threat of attacks by al Qaeda around Election Day, putting local law enforcement on alert days before the vote, officials said on Friday.

A U.S. government source in Washington said some federal agencies sent bulletins to local and state officials flagging the information but that the threat was relatively low level.

The Port Authority of New York and New Jersey, which operates airports, tunnels and bridges around New York City, continues high levels of patrol it has had in place, spokesman Steve Coleman said.

Coleman declined to offer details on the warning, but the New York City Police Department said the threat report lacked specifics and was still being assessed.

"We are aware of the information," the department said in a statement, adding that it was working with intelligence agencies and the Joint Terrorism Task Force.

Last month, the United States carried out strikes in Afghanistan targeting two of al Qaeda's senior leaders there, and al Qaeda's Yemen branch has posed a risk to merchant ships in waterways nearby.

U.S. intelligence agencies still view al Qaeda and its affiliates as a top counter terrorism priority.

The White House said it was aware of the reported al Qaeda threats and mindful of increased risk of attacks during events such as Election Day.

Texas Governor Greg Abbott said in a statement his office was monitoring the situation and urged Texans to remain vigilant. In Virginia, Brian Coy, a spokesman for Governor Terry McAuliffe, said: "We are doing everything we can to keep Virginians safe, and we're confident they are going to be able to vote safely on Election Day."

-Reuters