The Federal Bureau of Investigation advised US law enforcement agencies and private security firms to be on the alert in light of threats by Islamic State to target churches and Christmas celebrations, media reports said.
This move by security forces came days after an Islamist militant assassinated the Russian ambassador to Turkey and after the deaths of a dozen people when a truck rammed into a Christmas market in Berlin.
IS claimed responsibility for the attack in Germany.
The call for vigilance by the FBI on Friday follows online messages from IS urging its sympathizers in the US to attack Christian houses of worship.
"The FBI is aware of the recent link published online that urges attacks against US churches. As with similar threats, the FBI is tracking this matter while we investigate its credibility," a FBI bulletin said.
Federal officials said they have no credible information on specific threats.
The man identified by German authorities as the driver of the truck that crashed into the market, Tunisian national Anis Amri, died Friday in a shootout with police in Milan, Italy.
(With IANS inputs)
