US authorities have warned police across the nation that Islamic State sympathisers are continuing to call for attacks on churches and other Christmas gathering sites.
The warning was issued after a publicly available list of US churches was posted on a militants' social media site. It also came just days after Monday's attack at a Christmas market in Berlin that killed 12 and injured 56.
Separately on Friday, police in Australia detained five men suspected of planning a series of Christmas Day attacks using explosives, knives and a gun in the heart of Melbourne.
FBI spokesman Andrew Ames said US citizens are advised to maintain awareness of their surroundings and report suspicious activity.
He said the FBI will work closely with federal, state and local police should there be any potential threat to public safety.
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"The FBI is aware of the recent link published online that urges attacks against US churches," Ames said.
"As with similar threats, the FBI is tracking this matter while we investigate its credibility."
