Some 629 migrants, including 11 children and seven pregnant women, have been afloat in the central Mediterranean aboard the Aquarius rescue ship since Sunday, when both Italy and Malta refused to let them dock.
Spain unexpectedly offered on Monday to take in the group of mainly sub-Saharan Africans, who were picked up off the Libyan coast over the weekend.
But the Aquarius is heavily overcrowded, making the four-day trip to Spain particularly perilous.
To resolve the problem, two Italian boats moved alongside the Aquarius on Tuesday to share out the migrants before heading west through what were forecast to be stormy seas.
The convoy set sail at around 9:00 pm local time, according to a spokeswoman for the Franco-German charity SOS Mediterranne which is operating the Aquarius.
It will take the Aquarius about 10 days to make the trip to Spain and back, leaving the Dutch-flagged Sea Watch 3 alone off the coast of Libya - a staging ground for people smugglers - looking out for migrant boats in distress.
The Sea Watch reported that a migrant shipwreck had claimed at least 12 lives.
A U.S. Navy ship, Trenton, radioed Sea Watch on Tuesday to say it had picked up 12 bodies and 41 survivors from a sinking rubber boat.
“This shows what happens when there are not enough rescue assets at sea,” Sea Watch spokesman Ruben Neugebauer said.
Italy has taken in more than 640,000 mainly African migrants over the past five years.
Other EU states have largely ignored pleas by Rome to take in some of the newcomers and share the cost of their care, heightening anti-European and anti-migrant sentiment in Italy.
Matteo Salvini, Italy’s new interior minister and head of the far-right League, has said his decision not to accept the migrant boat is aimed at forcing other European states to help bear the strain.
However, French President Emmanuel Macron denounced the decision to block the Aquarius, saying that under international law Italy should have taken the migrants in.
“There is a degree of cynicism and irresponsibility in the Italian government’s behavior with regard to this dramatic humanitarian situation,” government spokesman Benjamin Griveaux quoted Macron as telling his cabinet.
Gabriel Attal, a spokesman for Macron’s party, went further, telling Public Senat TV: “The Italian position makes me vomit.”
Italian Prime Minister Giuseppe Conte responded angrily.
“Italy cannot accept hypocritical lessons from countries that have always preferred to turn their backs when it comes to immigration,” Conte said in a statement.
Italy also received backing from Hungary’s right-wing Prime Minister Viktor Orban, who is a friend of Salvini and is known for his fiercely anti-immigrant rhetoric.
“It was so depressing to hear for years that it is impossible to protect maritime borders,” Orban told reporters in Budapest.
“Willpower has returned to Italy.”
Even as Italy dispatched the charity ship, an Italian coast guard vessel with 937 migrants aboard was heading north from the Libyan coast and was expected to dock in Sicily on Wednesday.
“No one should dare brand Italy or its government as inhumane or xenophobic,” Transport Minister Danilo Toninelli told Radio Capital.
The Sea Watch met the Trenton and was asked to pick up the survivors and bodies, but the group refused to bring them on board without a written guarantee from Italy.
“We have the capacity to take them on board, but we will only do so if there is a written statement by Italian authorities that we will be able to disembark them within 36 hours,” Neugebauer said.
“We cannot go to Spain.”
Doctors Without Borders (MSF), which is operating the Aquarius alongside SOS Mediterranne, urged Rome to drop plans for the lengthy trip to Spain for its migrant passengers.
“This plan would mean already exhausted rescued people would endure four more days travel at sea,” it said on Twitter.
“MSF calls for people’s safety to come before politics.”
Salvini’s League scored its best-ever result in March national elections, partly on pledges to deport hundreds of thousands of migrants and halt the flow of newcomers, and has formed a coalition with the anti-establishment Five Star Movement.