It is perhaps "only a matter of time" before Rome is hit by a Barcelona-style attack but security forces are ready in case the Vatican is targeted, the head the Swiss Guard has said.

Security has been stepped up at religious sites throughout Italy, including at the Vatican, since last year, when a truck driven by a suspected Islamist militant killed 86 people in the French city of Nice.

Barriers and police and army vehicles have been placed around St. Peter's Basilica to make it harder for a vehicle to gather speed in an attack such as the one last week in Barcelona, which killed 13 people.

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Despite threats from Islamic State, Rome and other Italian cities have so far been spared the kind of vehicle attacks that have also hit Nice, London, and Berlin.

"It could perhaps be just a matter of time before there is such an attack in Rome, but we are prepared," Christoph Graf, the commandant of the Swiss Guard, was quoted as telling the Swiss Catholic website Cath.ch.

Graf, referring to the attack in Barcelona, spoke on the sidelines of a religious ceremony in the Swiss city of Solothurn earlier this week.

Websites linked to Islamic State militants have made threats against Catholic targets in Rome in recent years.

In 2015 in a video showing the beheading of 21 Egyptian Coptic Christians in Libya, one of the killers said: "Safety for you crusaders is something you can only wish for ... We will conquer Rome, by the will of Allah."

At about the same time a website used by militants ran a photo montage showing the movement's black flag flying from the obelisk at the centre of St Peter's Square.

The Swiss Guard has its origins as a papal protection force in the 16th century and numbers about 110 men, all of Swiss nationality.

It shares responsibility for the protection of the pope and the Vatican with a police force of about the same size. Italian police are responsible for patrolling the Vatican's external perimeter in Rome.

Both Vatican security forces are trained in anti-terrorism tactics and in the use of modern weapons.