Diverting the rescue ship to Spain - some four days' away, with poor weather forecast - keeps the Aquarius away from the area where its search-and-rescue capacity is most needed, in worldwide waters off the Libyan coast, putting other lives at risk.
Italy summoned the French ambassador for consultations Wednesday after France accused the new populist Italian government of cynical, irresponsible behaviour by refusing entry to a migrant ship, evidence that the standoff is having continent-wide repercussions.
The jibe comes as Italian Prime Minister Giuseppe Conte considered postponing a meeting with French President Emmanuel Macron, who said Rome had acted with "cynicism and irresponsibility". Spain stepped up and offered the Aquarius safe harbor after Italy and Malta both refused.
The vessel is on its way to Valencia, Spain - following Italy's decision to close its ports to the migrants rescued this past weekend in the Mediterranean Sea.
The standoff over the Aquarius appeared a clear tactic by Italy's new government to force Europe's hand at the upcoming summit of European Union leaders in Brussels June 28-29.
The French government was more conciliatory Wednesday, when the Foreign Ministry said France recognized the burden Italy has carried.
United Nations refugee chief Filippo Grandi urged the European Union on Wednesday to sort out its "shameful" refugee policy after both Italy and Malta turned away a ship carrying hundreds of migrants.More news: Saudi-led coalition launches attack on Yemen’s port city
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However, the Sea Watch is still waiting to pick them up after asking for a written guarantee that it will be allowed to dock in an Italian port.
Austria's conservative Chancellor Sebastian Kurz, who since December has led a coalition government with the anti-migration Freedom Party, proposed the idea Wednesday of an "axis of the willing" - made up of Rome, Vienna and Berlin - to co-operate in tackling illegal immigration.
On Wednesday morning, an Italian coast guard vessel with 937 migrants and two bodies aboard docked in Catania, Sicily, while the Aquarius headed into increasingly rough seas on its way to Valencia, where it is due to arrive on Saturday.
"For me, the question of how we deal with the migration question is something of a litmus test for the future and the unity of Europe, and the interests of every country must be considered", she said.
Matteo Salvini accused France of turning back 10,000 migrants on its border with Italy. That has put pressure on Italy and Greece, the entry points for hundreds of thousands of people fleeing war and poverty.
More than 1.8 million migrants have entered Europe since 2014.
Grandi, an Italian national himself, pointed out that Italy had taken the brunt of the massive numbers of migrants crossing the Mediterranean from Libya towards Europe in recent years.