Police carrying out a "vast" anti-terrorist operation in Italy are concerned that members of terror groups like Islamic State and al Qaida may be hiding amongst the thousands of migrants that arrive on Europe's shores each week.
Governments across Europe are particularly worried about potential "sleeper" militants, apparently living normal lives in their countries, who may at some point in the future be activated to stage attacks at home or abroad.
Some of the Al Qaeda-linked suspects arrested in an anti-terror operation in Italy earlier may have been planning an attack on the Vatican, the Italian prosecutor has said.
Cagliari Chief Prosecutor Mauro Mura told reporters that as well as planning to launch attacks in Pakistan and Afghanistan, the suspects may also have been aiming to target the Holy Catholic City State.
Mura said that, in intercepted telephone calls, investigators had heard the suspects say they would launch a "big jihad in Italy," and other conversations suggested that also suggested a target might be the Vatican.
Italian police have mounted a "vast anti-terrorist" operation against a group reportedly inspired by al Qaida.
Reuters report that the organisation wanted to support attacks against Pakistan's government and US forces in Afghanistan.
A police official said that most of the 18 arrests were centered on the island of Sardinia, but the operation covered seven Italian provinces.
Some arrests have been made, but the operation is still "ongoing", the official said.
The police say that 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.
Up to 70 migrants, including young children, rescued from a dinghy in the Mediterranean after a gas cylinder on their boat exploded, have arrived in Sicily for urgent medical care.
ITV News correspondent John Ray reports from Sicily. Some viewers might find images in this video distressing.
It is the duty of EU member states to help deal with the thousands of migrants pouring across the Mediterranean, the commissioner for migration said.
Dimitris Avramopoulos said that while the EU is developing the policies, the member states and their resources must implement them.
"Europe is accomplishing its duty, it's up to the member states to implement it," he said, referring to a new EU policy to be presented in May.
He urged member states to show solidarity with those bearing the brunt of the arrivals.
His comments come as some 10,000 migrants have been rescued making the perilous crossing to Europe in the past week.
Dozens of migrants have been rescued from a dinghy after their boat's gas cylinder exploded, killing several and injuring others.Read the full story ›
Right-wing parties in Italy have voiced increasing anger at what they call an "invasion" of migrants heading to the country from Africa.
The government in Rome has called for help from regional authorities to provide beds for the thousands of new arrivals - but many politicians have been reluctant to comply.
Among them is Matteo Salvini, head of the Northern League party, who accused the government of running a "taxi service" to help people traffickers. Posting on Facebook, he added:
I ask the League's governors, mayors, assessors and councillors to say no, with every means, to every new arrival. The League is ready to occupy every hotel, hostel, school or barracks intended for the alleged refugees.
Stop the departures and prevent the landings, to save lives.
Head of centre-right party Forza Italia, Giorgia Meloni, has also said the boats setting off from north Africa to Italy "should be stopped as they leave", while Claudio Palomba, the prefect of Rimini, told the Telegraphthat the system was "close to collapse".
Official estimates suggest more than 30,000 people have crossed into Italy and Greece since the start of the year, making the treacherous trip across the Mediterranean to escape from Libya - including 10,000 picked up by search and rescue boats in the last week alone.
At least 900 more are believed to have drowned en route.
The tide of desperate migrants and refugees travelling from Libya to Italy shows no sign of stopping, with more overloaded boats being intercepted by authorities every day.
ITV News Correspondent John Ray reports from Sicily:
Police in Sicily have detained 15 Muslim migrants for allegedly throwing 12 Christians overboard during a fight at sea.
Palermo police learned of the incident while interviewing tearful survivors from Nigeria and Ghana, who arrived on Italian soil yesterday morning, after being rescued at sea.
The survivors said they had boarded a rubber boat April 14 at the Libyan coast with 105 passengers aboard.
During the crossing, Nigerian and Ghanaian Christians claim they were attacked by passengers from the Ivory Coast, Senegal, Mali and Guinea Bissau.
This development is the latest in a growing crisis as Italy struggles to cope with a sudden influx of tens of thousands of refugees escaping instability across Africa.
More than 500 new African migrants have arrived in the Western Sicilian port of Trapani.
The Italian Navy ship Comandante Foscari landed today with 586 arrivals - including women and children.
Italy has admitted it is struggling to cope with the thousands of migrants landing on its shores this spring.
These new arrivals come on the heels of a tragedy earlier this week, in which as many as 400 migrants died during the dangerous crossing between Libya and Italy after their boat capsized.