France warned Italy of "severe consequences" as it was set to take in on Friday a charity ship carrying more than 200 migrants rescued in the Mediterranean, who were denied entry by Rome.
The Ocean Viking ship had initially sought access to Italy's coast, which is closest to where the migrants were picked up, saying health and sanitary conditions onboard were rapidly worsening.
Italy refused, saying other nations must shoulder more of the burden for taking in the thousands of migrants trying to reach Europe from North Africa every year.
Prime Minister Giorgia Meloni, head of Italy's most right-wing government in decades, appears ready to push the dispute to the top of the European agenda.
French Interior Minister Gerald Darmanin slammed Italy's stance as "incomprehensible", saying the Ocean Viking "is located without any doubt in Italy's search and rescue zone".
He slammed Italian authorities for "making the migrants wait at sea for 15 days".
Later Thursday, the Italian government in turn used the same word -- "incomprehensible" -- to describe France's response to allowing a migrant ship to disembark in a French port.
Interior Minister Matteo Piantedosi said the request had been for "234 migrants, when Italy has taken in 90,000 just this year".
After seeing dozens of requests to dock refused, the charity operating the ship, SOS Mediterranee, this week turned to France for help, though without any official government response initially.
France said it would evacuate three migrants needing urgent medical care, with a helicopter taking them and a caregiver to a hospital in Corsica.
Darmanin said the 230 remaining migrants on the ship, including 57 children, would be welcomed at the Toulon military port on Friday, an "exceptional" decision that would not guide future action.
Nine European nations have committed to host two-thirds of the migrants, he told TF1 television later on Thursday, with the remaining third staying in France.
Germany will take "more than 80", while Croatia, Romania, Bulgaria, Lithuania, Malta, Portugal, Luxembourg and Ireland will also contribute in the name of "European solidarity", he added.
In retaliation, France has suspended a plan to take 3,500 refugees currently in Italy, part of a European burden-sharing accord, and urged Germany and other EU nations to do the same.
"There will be extremely severe consequences for bilateral relations and European relations," Darmanin warned, adding that French police would also reinforce controls at Italian border crossings.
- Strained solidarity -
The flare-up of tensions echoes European migrant disputes four years ago, when French President Emmanuel Macron in particular clashed with Italy's populist interior minister Matteo Salvini.
Salvini, recently returned to government as Meloni's deputy, responded to France's decision to halt the migrant-sharing deal with the sarcastic tweet: "European solidarity."
France had insisted that under international maritime law, Rome had to take in the Ocean Viking and the 234 distressed migrants it rescued, not least after it granted access this week to three other rescue ships carrying around 700 people.
But Italy's Foreign Minister Antonio Tajani said this week that he was sending 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", Tajani said ahead of a meeting of EU ministers next week.
In June, around a dozen EU countries, including France, agreed to take in migrants who arrive in Italy and other main entry points.
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 percent arrived after being rescued by NGO vessels, according to the Italian authorities.
- 'Relief and anger' -
SOS Mediterranee said it felt "a mixture of relief and anger" after France's agreement to let it disembark.
"Disembarking almost three weeks after their rescue... is the result of a dramatic failure from all the European states, which have violated maritime law in an unprecedented manner," the charity's director of operations, Xavier Lauth, said in a statement.
According to the UN's International Organization for Migration, 1,891 migrants have died or disappeared so far this year while trying to cross the Mediterranean, often with help of smugglers crowding them into rickety vessels.