FBI issued a warning to state and city police ahead of the Christmas weekend

They warned of possible attacks on churches after list was posted online

Bureau said no one threat had been traced but that officers should be vigilant

It comes less than a week after attack by a lone fanatic on Berlin Christmas markets

The FBI has warned state and city police of possible ISIS threats against the US after a list of churches across the country was posted online by fanatics.

The bureau said while no one specific threat had been traced, officers should be more vigilant over the holidays given the amid 'aspirational calls for attacks'.

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In a memo it told how the Islamic extremist group had compiled a list of churches across the country and shared it online.

'The FBI is aware of the recent link published online that urges attacks against U.S. churches.

The FBI said while no one specific threat had been traced, police officers should be more vigilant

'As with similar threats, the FBI is tracking this matter while we investigate its credibility,' it said.

It comes less than a week after an attack at Berlin's Christmas markets by Tunisian fanatic Anis Amri.

He plowed a hijacked truck into crowds at Kaiser Wilhelm Memorial Church, killing 12 people and injuring dozens more.

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Amri was shot dead in Milan on Friday in the early hours of the morning by two young policemen carrying out routine stops.

ISIS claimed responsibility for the massacre shortly after it unfolded, describing Amri as a 'soldier'.

They praised him for targeting 'nationals of the coalition countries'.

The US has been at the forefront of Western efforts to disarm the terrorist group in Syria and Iraq since 2014.

The warning comes less than a week after a Tunisian fanatic plowed a hijacked truck into crowds at Berlin's Christmas market, killing 12 people. ISIS later claimed responsibility for the attack

There have not been any orchestrated ISIS attacks on US soil. Several incidents involving one or two shooters or attackers have however been traced back to the terrorist organization.

Among them is the slaughtering of dozens of party-goers at Pulse Nightclub in Orlando in June, 2016.

Gunman Omar Mateen took hostages in the gay nightclub after spraying dancing crowds with bullets.

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Forty nine people were killed in the attack. In a 911 call, Mateen pledged allegiance to ISIS.