St Peter's Basilica in Rome, along with Milan's cathedral and La Scala opera house, could be "potential targets" for future Isis attacks, the US State Department has warned.
Other targets could include "general venues" such as churches, synagogues, restaurants, theatres and hotels.
Italian law enforcement are currently "working to identify five people" identified by the FBI, according to Paolo Gentiloni, Italy's foreign minister.
The news comes as the Iraqi branch of Isis posted a video threatening fresh suicide and car-bomb attacks, titled Paris Before Rome.
They particularly mention attacks against France and the US.
"We will end with what is called the White House and we will blacken it with our fire," one militant says in the video.
In a security message to US citizens living in Italy, the US embassy said: "Terrorist groups may possibly utilise similar methods used in the recent Paris attacks."
"The Italian authorities are aware of these threats," it added.
Following last week's attacks, around 700 soldiers are being deployed in Rome for added security.
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