A German charity rescue ship was impounded and the captain placed under house arrest after defying Italian authorities and docking in Lampedusa with 40 migrants on board, causing a growing diplomatic spat between Italy and Germany.

Italian Interior Minister Matteo Salvini, a sharp right-wing critic of the migrant rescues, said the aggressive tactics of Sea-Watch 3 captain Carola Rackete made it clear that Italy was dealing with "criminals." She faces up to 10 years in prison for her actions. She also risks a fine as high as 50,000 euros ($58,000) under a recent Salvini-backed law cracking down on private rescue vessels. Any fine might be covered by a reported 100,000 euros supporters in Italy recently donated to help Sea-Watch. Salvini slammed the captain's defiance, branding her actions as tantamount to an "act of war."

The migrants had been rescued from an unseaworthy vessel launched by Libya-based human traffickers but Salvini had refused to let them disembark on Lampedusa until other European Union countries agreed to take them. Five nations pledged to do so on Friday: Finland, France, Germany, Luxembourg and Portugal.

Meanwhile, the next stand-off could be looming. German charity Sea-Eye and Spain's Proactiva Open Arms have sent two rescue ships to the Mediterranean off Libya's coast. Directing his comments at the two charities, Salvini said: "Do what you want, but do not take us for fools."