The Department of Homeland Security and Federal Bureau of Investigation sent a bulletin to law enforcement agencies across the U.S. on Friday urging vigilance against lone actor attacks.

U.S. officials are not aware of any credible threat. However, officials told CBS News the bulletin was issued after the Islamic State of Iraq and Syria (ISIS) made an online call for attacks against churches in the U.S.

ISIS’ call -- and authorities’ bulletin urging vigilance -- followed the Monday assassination of Russia’s ambassador to Turkey and a fatal truck attack on a Christmas market in Berlin for which the terror group claimed responsibility.

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“The FBI is aware of the recent link published online that urges attacks against U.S. churches,” the bullet said. “As with similar threats, the FBI is tracking this matter while we investigate its credibility.”

“The FBI asks members of the public to maintain awareness of their surroundings and to report any suspicious activity to law enforcement,” the bulletin said.