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The FBI is probing a major terror attack planned for the day before the US elections in multiple states, according to reports.

It is thought that al-Qaeda is plotting to strike in Texas, New York and Virginia but specific locations are not yet known.

The threat comes just four days before the polls officially open ending the closely-fought race for the White House between Hillary Clinton and Donald Trump.

An FBI spokesman told CBS News: "The counterterrorism and homeland security communities remain vigilant and well-postured to defend against attacks here in the United States.

"The FBI, working with our federal, state and local counterparts, shares and assesses intelligence on a daily basis and will continue to work closely with law enforcement and intelligence community partners to identify and disrupt any potential threat to public safety."

The terror alert comes as the US beefs up its cyber defences amid fears hackers could target electronic voting systems on polling day.

(Image: MANDEL NGAN/AFP/Getty Images)

The Department of Homeland Security is probing and scanning election systems for vulnerabilities in the run-up to the vote.

Cyber security experts and US officials say chances that a hack could alter election outcomes are remote, in part because voting machines are typically not connected to the internet.

But the FBI sent a flash alert in August to states after detecting breaches in voter registration databases in Arizona and Illinois.

The potential for violence around the election has loomed in the background of the campaign for months.

Armed groups around the country have pledged in unprecedented numbers to monitor voting sites for signs of election fraud.

(Image: Barcroft Media)

Voter intimidation reported at polling sites so far prompted Democrats to accuse Trump of a "campaign of vigilante voter intimidation" in four states on Monday.

It comes after a 'homemade bomb' hidden in a trashcan exploded leaving 29 people injured in Manhattan in September.

Terrified witnesses said they heard a "loud boom" and felt the ground beneath them shake as a huge "fireball" erupted into the air.

(Image: Barcroft Media) (Image: REUTERS/Stephanie Keith)

A second suspected bomb in a pressure cooker with wires attached to it and connected to what resembled a mobile phone was found blocks away.

Hours later police descended on a train station in New Jersey and faced another explosion while dealing with a suspicious package.

The FBI are probing whether a terrorist sleeper cell was behind the bombings after prime suspect Ahmad Khan Rahami was captured alive following a bloody shootout with police.

New York City mayor Bill de Blasio confirmed the terrifying blasts were an "act of terror" but declined to comment on the suspect's motivation.