By Agence France-Presse
Dozens of people have been blocked at the Sicilian port of Catania on the Diciotti vessel since Monday night because the Italian government is refusing to allow them to disembark without commitments from the EU to take some of them in.
Italy on Friday said it would pull its funding for the EU as a "compensatory measure" if the bloc refused to come forward and help with relocating the migrants.
The UNHCR on Saturday said EU member states should "urgently" offer places to some of the migrants on the vessel, adding: "In the meantime, UNHCR urges Italian authorities to allow the immediate disembarkation of those on board".
A high-level meeting of a dozen EU member states in Brussels on Friday, held to discuss what officials said was the broader issue of the disembarkation of migrants rescued at sea, failed to produce an immediate solution for the Diciotti migrants.
"The European Union has decided to turn its back on Italy once again," Deputy Prime Minister Luigi Di Maio wrote on his Facebook page, adding that his country had no choice but to "take a compensatory measure in a unilateral way... we are ready to reduce the funds that we give to the European Union".
"They want the 20 billion euros ($23 billion) paid by Italian citizens? Then let them demonstrate that they deserve it and that they are taking charge of a problem that we can no longer face alone. The borders of Italy are the borders of Europe," he added.
Migration is a hot-button issue in Italy, where hundreds of thousands of people have arrived since 2013 fleeing war, persecution and poverty in the Middle East, Africa and Asia.
Under EU rules people must seek asylum in their country of arrival, but Italy's new government has increasingly barred boats from docking at its ports.
Brussels quickly hit back at Di Maio's comments.
"Unconstructive comments, let alone threats, are not helpful and they will not get us any closer to a solution," European Commission spokesman Alexander Winterstein told a briefing.
"The EU is a community of rules and it operates on the basis of rules, not threats."
No deal was struck about the Diciotti migrants at the talks, as a source at the European Commission said "this was not a meeting where decisions were taken".
However, the source said they discussed "the need for a shared and rapid solution for the migrants on board of the Diciotti as well as those most recently disembarked in Spain and Malta."
EU figures for 2016 say Italy contributed just under 14 billion euros to the EU budget -- less than one percent of its gross national income -- while the bloc spent 11.6 billion euros in Italy.
Di Maio, who heads the anti-establishment Five Star, said Italy didn't want the "mickey taken out of us by the union's other countries" on the distribution of migrants.
"The EU was born of principles like solidarity. If it is not capable of redistributing 170 people it has serious problems with its founding principles," he said in an interview with state broadcaster RAI.
Prosecutors from Sicily were travelling to Rome to question officials, including Italy's hardline Interior Minister Matteo Salvini, about the illegal detention of those onboard.
"If a judge wants to arrest me, I expect it, no problem," Salvini said Friday.
'We've had enough'
Salvini stopped the majority of the migrants disembarking from the ship after they were rescued on August 15.
His only concession was to allow 27 unaccompanied minors off the boat Wednesday.
Opinion polls suggest that Salvini's stance has boosted his far-right League party's approval rating to around 30 percent -- a more than 10 point jump from its showing in March's election -- and is now level with the Five Star Movement with which it has governed Italy since early June.
However, according to Salvini's own ministry, migrant arrivals are more than 80 percent down on the same period last year, with just over 19,500 arriving up to August 23, compared to 98,000 in 2017.
In France, meanwhile, the presidency called for a "co-ordinated, long-term European mechanism" to distribute migrants that would include Italy. "There are forces in Italy that are looking to co-operate, we want to believe Italy wishes to play the game," the Elysee palace said.