Interior Minister Matteo Salvini on Wednesday denounced a judge who freed the German captain of a charity rescue ship accused of forcing an Italian naval blockade and suggested the ruling was politically motivated.

The national magistrates association warned that Salvini was undermining the judiciary's independence in a case that has whipped up diplomatic tensions in Europe and threatens to compromise Salvini's efforts to curb migrant flows from Africa.

In a surprise ruling, Judge Alessandra Vella freed Carola Rackete from house arrest on Tuesday, saying that by bringing migrants to port in defiance of the Italian navy she was looking to save lives, which trumped all other concerns.