The Italian Coast Guard detained a migrant rescue vessel belonging to a German NGO for not complying with security rules, they said.
"An inspection revealed various irregularities of a technical nature likely to endanger not only the safety of the crew members but also that of the persons who were or who could in the future be recovered on board during operations of help," the Coast Guard said in a statement.
The Coast Guard said that the organisation had life-saving equipment sufficient for a maximum of just 27 people, stating that the crew could not guarantee the evacuation of people on board.
The ship Sea-Eye 4 had disembarked more than 400 migrants in the Sicilian port of Pozzallo and was quarantining as required by health authorities.
The German NGO denounced the decision of the Italian authorities to detain their rescue vessel.
"What kind of Europe is it, in which it is possible to arrest a rescue ship because it has saved too many people? I claim that if there were 408 Europeans we would be free," Sea-Eye Chairman Gorden Isler said on Twitter.
Isler said the ship's crew had the same day been made honorary citizens of Palermo.
"The argument is still that German rescue ships routinely save too many people from drowning and are incorrectly certified for this humanitarian purpose," Isler said in the statement.
"Our captain fulfilled his duty of rescue at sea in an exemplary manner. He saw emergency situations at sea and carried out a rescue in complete safety. EU states can draw inspiration from this."