A rescue vessel carrying 629 migrants is stranded in the Mediterranean after Italy’s new interior minister refused permission for it to dock.
Matteo Salvini, leader of the right-wing League, said Malta should accept the Aquarius but he was rejected.
Malta says the German charity SOS Méditerranée picked up the migrants in Libyan waters, which means they fall under Italy’s jurisdiction.
Italy is the main entry for migrants crossing from North Africa to Europe.
The League promised voters during Italy’s recent general election that it would take a tough stance on immigration.
SOS Méditerranée, which runs the Aquarius, said the ship had been instructed by the Italian Maritime Rescue Coordination Centre to stand by in its current position, 35 nautical miles from Italy and 27 nautical miles from Malta.
It reports that the migrants were picked up in six different rescue operations off Libya’s coast.
“Our objective is the disembarkation in a port of safety of the 629 people now on board the Aquarius – some we rescued yesterday night in difficult conditions,” charity spokesperson Mathilde Auvillain was quoted as saying by Reuters news agency.
The UN refugee agency in Italy has called on “states and actors involved” to “rapidly find solutions to allow migrants and refugees on board the Aquarius to disembark safely and quickly”.
Many of the migrants were rescued in waters controlled by Libya, a common route for undocumented migrants attempting to enter Europe from sub-Saharan Africa.
Some 400 were rescued by the Italian Navy, the Italian coastguard and merchant vessels before being transferred to the Aquarius.
Among those saved and brought on board the Aquarius are 123 unaccompanied minors, 11 younger children and seven pregnant women, SOS Méditerranée says.
The minors are aged between 13 and 17 and come from Eritrea, Ghana, Nigeria and Sudan, according to a journalist on the ship, Anelise Borges.
Tweeting during the night, she added that the situation on board was calm and most of the people rescued were asleep.
One Aquarius crew member, Alessandro Porro, told Italian news channel Sky TG24 that they desperately needed to know which port to go to.
“The people we saved yesterday were in a difficult condition, at least 50 were at risk of drowning,” he added.
He said on Sunday that Italy was saying “no to human trafficking, no to the business of illegal immigration”.
“Malta takes in nobody,” he said. “France pushes people back at the border, Spain defends its frontier with weapons.”
“It is not possible for Malta to say ‘no’ to every request for help. The Good Lord put Malta closer than Sicily to Africa.”
Last week, Mr Salvini said Rome should increase its deportations of migrants and the Italian government also wants to relocate asylum seekers EU-wide – a scheme already rejected by some member states.
Mr Salvini says he is considering action against organisations rescuing migrants at sea. He has previously accused them of being in cahoots with people-smugglers.
The government’s critics say its plan to repatriate migrants is unworkable and risks fomenting racism and politicising a humanitarian issue.
A controversial deal between Italy’s former government and authorities in Libya has led to a drop in overall arrivals since last summer but Italian officials say 13,500 migrants have been registered so far this year.
A spokesman for the Maltese government told AFP news agency Malta was “neither the coordinating nor the competent authority” in the rescue operation.
Mr Salvini earlier accused the Maltese government of not doing its fair share when another rescue ship, the Seefuchs, was stranded in volatile seas with 126 migrants on board.
Malta reportedly refused to send assistance to the vessel and it was allowed to dock in the Sicilian port of Pozzallo on Saturday, the non-governmental organisation Sea Watch reported.
Malta’s government insisted it had adhered to all its obligations regarding immigration.