After Sanchez's move, Italian Prime Minister Giuseppe Conte thanked Spain - as did Maltese Prime Minister Joseph Muscat while also blaming Italy for the ship's prolonged journey.
The Aquarius, a former North Atlantic fisheries protection ship now used by humanitarian groups SOS Mediterranee and Medecins Sans Frontieres (Doctors without Borders), is seen in December 2017 during a rescue operation in the Mediterranean Sea.
Matteo Salvini, in a joint statement with Danilo Toninelli, the minister in charge of the Italian coastguard, insisted that Malta "cannot continue to look the other way when it comes to respecting precise worldwide conventions on the protection of human life".
Salvini, leader of the right-wing League, had promised voters ahead of the country's recent general election that it would act tough against immigration.
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But Salvini, who channeled Trump last week by telling reporters at a refugee camp in Sicily that Tunisia "isn't exporting gentlemen, it seems more often they're exporting convicts", responded defiantly to calls for mercy, even from the Vatican.
His tweet concluded with the hashtag #chiudiamoiporti - which means "we're closing the ports" in Italian. The trip would take four to five days, and with so many people aboard the boat and bad weather making seafaring conditions hard, going to Valencia would be "a insane idea".
SOS Mediterranee, the charity co-operating the migrant ship, said it was awaiting instructions about where to disembark from Italy's coastguard, which coordinated the sea rescues.
"Italy is done bowing its head and obeying", Salvini tweeted on Mondy.
In another Facebook post on Monday, Salvini assailed the Sea-Watch 3, another rescue ship operating in Mediterranean waters that had to wait hours to be assigned a disembarkation port over the weekend.
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Meanwhile a spokesman for the Maltese government said that "Malta was neither the coordinating nor the competent authority" in the rescue.
Italy's spat with Malta began after its reported refusal to come to the aid of another rescue ship Seefuchs, which was stranded with 126 migrants on board due to violent seas until it was allowed to dock at the Sicilian port of Pozzallo on Saturday. It added that the Aquarius "is still waiting for definitive instructions regarding the port of safety".
In defiance of Salvini and the government's anti-immigrant stance, several Italian mayors have suggested they would be prepared to welcome the Aquarius into their port. "We need to know who is arriving", a local man, Salvo Cavallaro said, "these 600 people, we don't know who they are".
"We would need a resupply at sea which is not so easy to organize so we urge Italy to find a solution very soon close to our position", he told The Associated Press in Paris.
Both the mayors of Valencia and Barcelona offered to take the boat in at their ports.
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The Mediterranean has always been a key route into Europe for refugees and migrants are using North African states, such as Libya, Tunisia, and Egypt as launch points for journeys. Malta said Italy co-ordinated the rescues and that the tiny island nation - which in the last few years has only accepted a few hundred migrants - has had nothing to do with it.