Briefing journalists after a cabinet meeting, government spokesman Benjamin Griveaux said President Emmanuel Macron had made clear that the country with the nearest coastline to a stranded ship bore responsibility under maritime law.
"There is a degree of cynicism and irresponsibility in the Italian government's behaviour with regards to this dramatic humanitarian situation," Griveaux quoted Macron as saying.
The 629 migrants, including 11 children and seven pregnant women, have been drifting in the central Mediterranean aboard the Aquarius since Sunday, when both Italy and Malta shut their ports to it.
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 are forecast to be stormy seas.
The convoy set sail at around 9 p.m. (1900 GMT), 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.
