ROME: Italian police on Thursday arrested five Tunisians as part of what they said was a major operation against suspected supporters of terrorism in central and southern Italy.

The swoop is the latest in a series of similar raids this month, coming as Italy steps up the number of foreigners it expels.

On Wednesday anti-terrorism police arrested an Italian citizen of Moroccan origin who they said was an Islamic State sympathiser planning a truck attack.

"A vast anti-terrorism operation" was being carried out by special investigation forces based in Rome and the nearby town of Latina, police said in a statement on Thursday.

The five Tunisians detained are suspected of being part of a network connected to Anis Amri, the Tunisian who killed 12 people when he drove a truck into a Christmas market in Berlin in December 2016, judicial sources told Reuters.

Amri was shot dead by Italian police near Milan four days after the Berlin attack.

Prosecutors suspect the arrested Tunisians of crimes including planning international terrorism, falsifying documents and helping illegal immigration of radicalised Tunisians, the sources said.

However, the sources said there was no evidence the suspects participated directly in the Berlin truck attack or that they were preparing any imminent attack in Italy.

As part of their operation, police were carrying out searches in Rome, the southern port city of Naples and the central and southern towns of Latina, Viterbo, Caserta and Matera, the sources said.

Interior Minister Marco Minniti said on Wednesday the security threat to Italy from Islamic State supporters was higher than ever because more foreign fighters were attempting to return to Europe via Italy after the jihadist group suffered defeats in Syria and Iraq.

So far this year, 29 foreigners, mostly Muslims, have been expelled from Italy as suspected threats to national security. </s> Italian police on Thursday arrested five Tunisians suspected of being part of a terror network tied to Anis Amri, who killed 12 people in a truck ramming attack at a Berlin Christmas market in December 2016.

Amri, a rejected asylum seeker from Tunisia, was shot and killed by police in Milan four days after the Berlin attack claimed by the "Islamic State."

One of the five Tunisian nationals arrested in the city of Latina, south of Rome, is suspected of having planned to provide false documents to get Amri out of Italy, police said.

Read more: Germany: How do terrorist groups compare?

In the wake of the Berlin attack, German authorities came faced intense criticism after it emerged Amri used multiple fake documents and should have been deported to Tunisia.

The anti-terror raids were carried out in Rome, Latina, Naples and other cities.

Reuters cited security forces as saying that the five Tunisians were not suspected of directly participating in the Berlin truck attack, nor were they imminently preparing one in Italy.

Italian authorities have carried out numerous counterterrorism operations this month, including one on Wednesday that led to the arrest of an Italian citizen of Moroccan origin suspected of planning a knife attack or truck ramming.

On Tuesday, police arrested what they said was a radical Islamic preacher indoctrinating young children in Foggia.

Interior Minister Marco Minniti has put security service on high alert for potential terror plots targeting tourist hotspots and other busy areas.

Italy is one of the few major European countries to be spared a large-scale terror attack in recent years.

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Italian authorities say a major terror alert in Rome last week was for an innocent man whose fiancé had made false, anonymous accusations to the Italian embassy in Tunisia.