Rescuing migrants in the Mediterranean may 'trigger' their departure from the coast of Libya, a boss at the EU border agency Frontex has warned.

Thousands of refugees have been killed making the perilous crossing on packed-out and unseaworthy boats heading for Italy.

Klaus Roesler, head of the Operations Division at Frontex, which manages the bloc's borders, says the organisation is now expecting up to 10,000 migrants a week to attempt the journey.

Rescuing migrants in the Mediterranean 'triggers' their departure from the coast of Libya, a boss at EU border agency Frontex has warned

According to the German newspaper Bild, Roesler said a possible reason for the large numbers willing to risk their lives on the crossing is the 'intense' search and rescue work now carried out by the EU.

He said this may unintentionally 'trigger departures' from war-torn Libya.

The United Nations' refugee agency estimates that over 48,000 migrants, most of them sub-Saharan Africans, have arrived in Italy since the start of the year in search of a better life in Europe.

A similar number made the treacherous sea journey over the same period last year.

But unlike previous years, new arrivals are increasingly finding themselves marooned in overcrowded camps in Italy as countries further north have shut their borders, effectively blocking their overland passage into the rest of Europe.

Earlier today, the EU gave its naval force in the Mediterranean the authority to search suspicious vessels at sea in a bid to stop arms getting to ISIS in Libya and to break up gangs smuggling migrants to Europe.

EU foreign ministers acted to boost the effectiveness of the five-frigate 'Sophia' mission after winning a U.N. mandate to reinforce an arms embargo on Libya, where ISIS is strengthening its grip, and limit the near-impunity of the people smugglers.

The United Nations' refugee agency estimates that over 48,000 migrants, most of them sub-Saharan Africans, have arrived in Italy since the start of the year in search of a better life in Europe

'We must act, both against those who exploit the migrants, those traffickers who exploit this misery, and against the arms trafficking that benefits Daesh,' French Foreign Minister Jean-Marc Ayrault said.

European military powers Britain, France and Germany say ending the chaos in Libya that has reigned since the fall of Muammar Gaddafi in 2011 is a crucial part of the EU's moves to end the migrant crisis.

NATO ships have also been sent to the Aegean to stem uncontrolled flows to Greece from Turkey.

Although EU ships in the central Mediterranean have picked up around 16,000 migrants at sea in the past year, their limited tasks of surveillance and information-sharing have meant they were not able to destroy weapons, catch traffickers or head off migrants trying to reach Europe by sea from Libya.

At least one smuggler vessel loaded with arms was allowed to pass an EU inspection in the Mediterranean in the past few months for lack of U.N. authority to act, one diplomat said.

Thousands of refugees have been killed making the perilous crossing on packed-out and unseaworthy boats heading for Italy

Now, the European Union also hopes NATO ships already patrolling in the central Mediterranean could link up with its 'Sophia' mission, providing intelligence about smuggling routes.

The United States has said it supports such a move.

Gangs, using profits from people smuggling into Europe, control arms networks stretching across Europe into North Africa via the Mediterranean.

Libya U.N. envoy Martin Kobler has told the Security Council that Libya is already awash with arms, with 20 million pieces of weaponry in the North African state of six million people.

By controlling new flows, the West could grant exemptions in the arms embargo to provide weapons to the U.N.-backed unity government in Tripoli and help it assert control in the lawless country.

'Getting control of illegal arms trafficking then gives the international community a lever, because we can consider relaxations in the arms embargo to allow certain groups access to ammunition,' said Britain's Foreign Secretary Philip Hammond.

Britain is expected to send another ship to the mission, as well as helicopters and other assets from 24 EU governments.