The EU is beginning a new operation in the southern Mediterranean to intercept boats smuggling migrants.
Under Operation Sophia, naval vessels will be able to board, search, seize and divert vessels suspected of being used for people smuggling.
Until now, the EU has focused on surveillance and rescue operations.
So far this year, more than 130,000 migrants and refugees have crossed to Europe from the north African coast. More than 2,700 have drowned.
However, many more migrants and refugees - mainly Syrians fleeing the country's civil war - are taking a different route. They are crossing overland into Turkey, before a short journey by sea to European Union member Greece and onwards to central and northern European countries, with Germany the preferred destination.
The migrant crisis was one of the issues raised by French President Francois Hollande and German Chancellor Angela Merkel in a rare joint address to the European Parliament in Strasbourg on Wednesday.
Analysis: James Reynolds, BBC News, Rome
The limits of the EU's mission are obvious. Its warships will have to stick to international waters - meaning that they will have to stay 12 nautical miles away from the Libyan coast.
It's not immediately clear whether or not six warships, together with support helicopters and drones, will make a practical difference to the number of journeys begun by smugglers and migrants.
On some days, about 20 migrant ships set sail from the Libyan coast.
The EU eventually hopes to move to a third, more aggressive phase of its operation - by operating within Libya's own territorial waters. But this will only be possible with the approval of either the UN Security Council or Libya itself.
The huge numbers of migrants reaching Europe have led to a crisis within the EU, with member states at odds on how to deal with the influx.
The deadliest voyages are from Libya to Italy.
The EU launched the first phase of its operation, called EUNavfor Med, in June, using naval surveillance to detect smugglers' boats and monitor smuggling patterns from Libya towards Italy and Malta.
Operation Sophia - the second phase - has been named after a baby born on an EU ship that rescued her mother off the coast of Libya in August.
From its headquarters in Rome, the operation's commander, Rear Adm Enrico Credendino, will oversee several EU warships in the Mediterranean, including the British frigate HMS Richmond.
His deputy, Rear Adm Herve Blejean, told the BBC that six ships were being used "to start to dismantle this business model by trying to apprehend some suspected smugglers".
Adm Blejean said he hoped four more vessels would soon join the operation, staffed with professionals who were ready to deal with any threats or resistance they might encounter.
A senior migration expert has warned, however, that the EU operation in the Mediterranean will fail unless the bloc also fights criminal gangs in Europe.
Eugenio Ambrosi, of the International Organization for Migration (IOM), said the "transnational criminal rings of traffickers are the same as those involved in weapons and drugs".
"The boats are not the reason for the smuggling," Mr Ambrosi said, adding that if efforts are just focused on the boats, "the nucleus of the criminals remains untouched".