Italian Prime Minister Matteo Renzi said Europe was witnessing "systematic slaughter in the Mediterranean".
"How can we remain insensible when we're witnessing entire populations dying at a time when modern means of communications allow us to be aware of everything?" Renzi said at a political event in Mantua.
On his way back to Rome, where he was expected to give a news conference later, Renzi spoke by telephone to French President Francois Hollande.
The leader of the anti-immigrant Northern League party, Matteo Salvini, who has made migration one of the centrepieces of his political agenda, called for a an immediate naval blockade of the coast of Libya.
The lawless state of Libya, following the toppling of former leader Muammar Gaddafi in 2011, has left criminal gangs of migrant smugglers a free hand to send a stream of boats carrying desperate migrants from Africa and the Middle East.
Around 20,000 migrants have reached the Italian coast this year, the International Organisation for Migration (IOM) estimates. That is fewer than in the first four months of last year, but the number of deaths has risen almost nine-fold.
In 2013, the previous government initiated the search-and-rescue operation "Mare Nostrum" or "Our Sea" after hundreds drowned in an incident off the coast of Lampedusa. The operation was cancelled last year, because of the cost and because some politicians said it encouraged migrants to depart by raising their hopes of being rescued.
That made way for the European Union's border control mission, Triton.
However Triton, which has a much smaller budget and narrower remit, has been criticised by humanitarian groups and Italy as inadequate to tackle the scale of the problem.