The charity ship Sea-Watch entered Italian waters on Wednesday with 42 migrants aboard, defying an order from Rome to stay away and provoking the ire of Interior Minister Matteo Salvini.
The captain of the German-owned boat, which flies the Dutch flag, had decided to head to the island of Lampedusa because the situation on board was "now more desperate than ever", the group said in a statement.
It said captain Carola Rackete felt that maritime emergency law permitted the ship to enter Italian waters.
In his first reaction, Salvini did not mince his words.
"They are not allowed to dock, I am ready to send the police ... this has broken my balls," Salvini said in a Facebook video. In a subsequent statement, he called the Sea-Watch "an outlaw ship" and said Rome had asked the Dutch government to assume responsibility for the migrants.
Shortly after the ship entered Italian waters without permission, Italian coastguard boats went out from Lampedusa to flank it.
According to a decree approved in June, the Interior Ministry has the power to deny access to territorial waters to vessels that it considers are a risk to security or public order, and to fine them.
Salvini, who heads the far-right League, has repeatedly accused charity rescuers of being complicit with people smugglers and has closed Italy's ports to their boats.
The Sea-Watch picked up 53 migrants from a rubber boat off the coast of Libya and had remained in international waters since June 12. Eleven migrants have already been taken off the ship by the Italian coastguard for health reasons.
"I Take Them To Safety"
Rackete, the ship's captain, defied the Italian government after the European Court of Human Rights ruled that Rome had no obligation to allow migrants to disembark from the vessel, although it was obliged to offer assistance at sea.
"I have decided to enter the port of Lampedusa. I know what I risk, but the 42 shipwrecked on board are exhausted. I take them to safety," she said on the ship's Twitter account.
An Italian magistrate ordered the Sea-Watch seized last month after it rescued dozens of migrants, accusing the crew of breaching immigration rules. It was released at the start of June and returned to the Libyan coastline.
Migrant arrivals to Italy have plummeted since Salvini took office a year ago, with 2,456 crossing the Mediterranean so far this year, according to official data, down 85% on the same period in 2018 and down 96% on 2017 levels