General view of St. Peter's Square as Pope Francis leads a ceremony for the canonization of four nuns, May 17, 2015. Reuters

The U.S. government has warned Italy that the Islamic State group could strike St. Peter's Basilica in Rome, Milan's cathedral and La Scala opera house and other "general venues," including churches, synagogues, restaurants, theaters and hotels. The U.S. Embassy in Rome said in a security message for U.S. citizens Thursday that "terrorist groups may possibly utilize similar methods used in the recent Paris attacks" that left hundreds dead or wounded.

The message from the State Department urged U.S. citizens to "remain vigilant and aware of their surroundings." It added: "The Italian authorities are aware of these threats."

The FBI has shared information with the Italians about possible threats, including at St. Peter's and the two Milan landmarks, Italian media reported. RAI state radio said early Thursday that "there are no direct signs of concrete threats." Foreign Minister Paolo Gentiloni told RAI that security services in the country were "working to identify five people."

Some 700 soldiers have been deployed across Rome for security after the terror attacks across Paris Friday night. At least 129 people were killed in the shootings and explosions, which were claimed by the Islamic State group.

Islamic State fighters threatened to carry out similar attacks in Rome and Washington in a video released Monday. “So we say to the countries that participate in the Crusader campaign: By Allah, you will have a day like that of France. By Allah, if we brought down France in its homeland, in Paris, then we swear, by the command of Allah, we will attack America in its heartland, in Washington, Allah permitting, and we will invade Rome, Allah permitting,” a man says on the video posted on social media sites.

French prosecutors confirmed Thursday that the mastermind behind the terrorist attacks in Paris was killed in a massive predawn police raid a day earlier. Abdelhamid Abaaoud was a Belgian national.