ROME -- At least ten people are being investigated in Italy for suspected ties with Islamist terrorist networks abroad, Italian media reported on Tuesday.

Chief prosecutor Massimo Pignatone in Rome opened a probe for possible subversive association with terrorist purposes on a dozen of foreign Muslim citizens long living in Italy, AdnKronos news agency reported.

Prosecutors would be looking into their possible ties with Islamic terrorist groups based abroad, and the formal investigation was opened after monitoring some suspicious activities on Internet.

The suspects would be considered as possible "lone wolves" and not part of a terror cell, according to ANSA news agency.

The recent terrorist attacks in France have prompt alarm in Italy, and the alert has been raised at the maximum level, head of the Rome section of the Italian police's anti-terror squad Diego Parente confirmed on Monday.

Security measures were tightened in the Jewish districts of major cities, and around possible sensitive targets such as media offices, embassies, public buildings, and places of worship.

Italian authorities also acknowledged the Vatican City in Rome was a "possible target," after media reports unveiled the United States have warned the seat of the Catholic Pope of possible acts of terrorism.

However, "no concrete signals of imminent attacks have emerged so far," interior minister Angelino Alfano declared.

On Monday, Rome-based Il Tempo newspaper reported some 211 people living in Italy would be overall considered by the anti-terrorism special unit as "at risk of having close ties with terrorist Islamic networks."