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".
Who is on the ship?
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.