ROME (Reuters, AFP, Bloomberg) - Italian police said Friday they had dismantled an Islamist network they have linked to Osama bin Laden, one of Pakistan's deadliest attacks and, more loosely, a possible plot to bomb the Vatican.

A total of 18 people were ordered arrested following a six-year investigation that began with a probe into an illegal immigration racket allegedly run from the island of Sardinia. Only nine of the suspects had been detained by Friday afternoon.

Prosecutor Mauro Mura told a press conference in Cagliari, Sardinia that members of the network had been in contact with two potential suicide bombers who came to Italy in 2010 and discussed the possibility of attacking the Vatican.

The men left Italy when they became aware they were under surveillance and the arrested suspects are not being investigated further on that score, Mura said.

Mario Carta, an officer in the DIGOS anti-terrorism unit that carried out the investigation, acknowledged there was no firm evidence of a conspiracy to kill the pope, only "strong suspicions" based on wiretapped conversations in which the suspects had spoken "in ironic terms" about the leader of the world's Catholics.

Vatican spokesman Federico Lombardi played down the significance of the incident. "This concerns a 2010 scenario that didn't materialise. It has no relevance today and there is no reason for particular concern," he told reporters.

All the suspects are Pakistanis or Afghans, Carta told Reuters earlier, adding that the operation was still in progress.

The alleged key figures in the network were Khan Sultan Wali, a shopkeeper and long-term resident of Olbia, Sardinia and an unidentified imam who carried out missionary work in Brescia and Bergamo in northern Italy, according to sketchy details provided by prosecutors at a press conference.

Some of the those arrested are responsible for past terror attacks in Pakistan including the 2009 Mina Bazar attack in Peshawar that killed more than 100 people, according to police statements. The group's aim was to push Pakistan to give up the fight against the Taleban and weaken support for the US in Afghanistan, police said.

European governments are increasingly worried about the threat of "sleeper" militants apparently living normal lives in their countries but who are then activated to stage attacks at home or abroad.

Police wiretaps had determined that two people among the 18 targeted by arrest warrants were suspected of being part of a group that had protected Al-Qaeda leader Osama bin Laden, who was killed by US special forces at his hideout in Abbottabad, Pakistan in 2011, a statement said.

The investigation, carried out by Italy's anti-terrorism DIGOS police branch, led to the arrest of an Imam in the northern city of Bergamo. He was suspected of being the spiritual leader of the group and had collected funds purportedly for religious purposes from Pakistanis and Afghans in Italy, police said.

The group supported the "armed struggle against the West", and wanted to incite a popular uprising against the Pakistani government so it would stop its backing of US forces in Afghanistan.

The United States has withdrawn most of its forces from Afghanistan. However, a relatively small number remains for training and special operations, while Washington is also carrying out drone strikes on Taleban militants.

The money was sent to Pakistan by members of the group who managed to avoid Italy's currency control regulations. In one case, 55,268 euros (S$80,017) were carried to Pakistan on a flight from Rome to Islamabad.

But police said much of the money was moved through the trust-based transfer system known as hawala, which pre-dates the time of the Prophet Muhammed, and which is the banking system of choice in Afghanistan's cash-based economy.

Some of those under investigation were believed to be involved in attacks in Pakistan, including one that killed more than 100 people in a market in the northwestern frontier city of Peshawar in 2009, the police added.

Most of the warrants centred on the Italian island of Sardinia, but the operation covered a total of seven Italian provinces, the statement said.

The organisation arranged for Pakistanis and Afghans to get into Italy under work contracts or as refugees seeking asylum and later sent some to cities in northern Europe, police said.