Italy is becoming increasingly frustrated at taking in the majority of migrants rescued trying to reach Europe from North Africa, and the row with France is its latest stand-off with other European nations.
It centres on the Ocean Viking, a charity ship that has sailed away from Sicilian waters toward France after unsuccessfully waiting for permission to dock in Italy since late October.
Run by the European charity SOS Mediterranee under a Norwegian flag, the ship has appealed to France to accept it as it sailed towards Corsica and possibly the French mainland, where it has also asked for access.
"It's a total blockage on the part of the Italians," SOS Mediterranee director Sophie Beau told AFP.
Ms Beau said the charity had lodged 43 official requests with no response.
As of Wednesday morning it "still had no official response" from French authorities, she said.
Italian leaders have claimed that France is ready to accept the migrants, but Paris has warned that the ship was in Italian waters and called the refusal to let her dock "unacceptable".
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It did not single out Italy or France by name, but noted the "clear and unequivocal" legal obligation to rescue distressed persons at sea.
"The situation onboard the vessel has reached a critical level and needs to be urgently addressed to avoid a humanitarian tragedy," the Commission said.
The stand-off echoes disputes four years ago between Italy and other EU nations, when French President Emmanuel Macron in particular clashed with Italy's populist, anti-immigrant interior minister, Matteo Salvini.
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The arrival of Giorgia Meloni at the head of Italy's most right-wing government in decades could again lead to strained ties that complicate decision-making at EU level.
"We're seeing diplomatic arm-wrestling between France and Italy that could open a breach for similar conflicts, because Italy is clearly challenging a European accord [on migrants] that was in its favour," said Matthieu Tardis of the French Institute for International Relations.
Ms Meloni on Wednesday defended the decision to allow only the most vulnerable migrants to disembark from three other NGO rescue ships in recent days.
She said they were "not shipwrecked but migrants", according to reports of comments in a closed-door meeting.
Italian health authorities later ordered the remaining migrants to be let off the boat as well, a choice "we found bizarre", Ms Meloni said, according to Ansa.
Italy's Foreign Minister Antonio Tajani claimed that France had offered Marseille as a port of safety for the Ocean Viking. Paris has not confirmed this.
Volker Turk, the UN High Commissioner for Human Rights, urged a rapid disembarkment and warned that "politics should not be pursued at the expense of people in distress".
"The migrants need our unconditional support — humanity demands it," Mr Turk said.
Under international law, ships in distress or carrying rescued passengers must be allow entry in the nearest port of call, which means Italy and often Malta are shouldering the burden of taking in those rescued after trying to cross the Mediterranean from Libya.
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In June, about a dozen EU countries, including France, agreed to take in migrants who arrived in Italy and other main entry points.
Mr Tajani said Rome's reluctance to offer its ports is a signal to EU nations that they must play an even bigger part.
Rome wants "an agreement to establish, on the basis of population, how migrants with a right to asylum are relocated to various countries", he said, before a meeting of EU ministers next week.
So far this year, 164 asylum seekers have been moved from Italy to other nations in the bloc that have volunteered to accept them.
But that is a tiny fraction of the more than 88,000 that have reached its shores so far this year, of which just 14 per cent arrived after being rescued by NGO vessels, the Italian authorities say.
According to the UN's International Organisation for Migration, 1,891 migrants have died or disappeared while trying to cross the Mediterranean so far this year.