FBI
The FBI authorities also said that all the citizens in the US were advised to be aware of their surroundings.Reuters

The United States Federal Bureau of Investigation (FBI) officials on Friday warned of possible Islamic State (ISIS) attacks across the country on churches and holiday gathering sites over the Christmas season.

The federal authorities said that the ISIS sympathisers were continuing their call for attacks at various sites in America. The German capital on Monday suffered an attack of a similar kind proposed by ISIS supporters where an ISIS terrorist took over a truck and rammed the vehicle into a crowd gathered at the Christmas market in Berlin. The attack killed at least 12 people and injured 68 others.

Shortly before the warning, United States president-elect Donald Trump took to Twitter on Friday and said that the Berlin attack was a "purely religious threat." He wrote, "The terrorist who killed so many people in Germany said just before crime, 'by God's will we will slaughter you pigs, I swear, we will slaughter you. This is a purely religious threat, which turned into reality. Such hatred! When will the US, and all countries, fight back?"

The FBI issued a warning after a publicly available list of US churches was posted on a militant social media site.

"As part of the continuous dialogue with our law enforcement partners, the FBI routinely shares information about potential threats to better enable law enforcement to protect the communities they serve," FBI spokesperson Andrew Ames said.

The authorities also said that all the citizens in the US were advised to be aware of their surroundings and report any kind of suspicious activity they witness around them.

"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. We continue to work closely with our federal, state and local law enforcement partners should there be any potential threat to public safety," Ames added.

The Berlin attack suspect from Tunisia, Anis Amri, was shot dead in Milan on Friday by the Italian police in a shootout.