After a flurry of boat departures that sent hundreds of migrants to their deaths in the Mediterranean, survivors told police they had been kept for weeks on one meal a day in holding houses near the Libyan shore.

Then they boarded the rubber or wooden vessels, but only those co-opted to run or drive the boats were given life-jackets, according to accounts given to Italian police.

As calmer summer weather begins, European officials who struck a deal with Turkey to block crossings to Greece have been scrambling for ways to shut down flows on the other major sea route into the EU from Libya.