An Italian prosecutor said last month that charity boats were colluding with traffickers off Libya, in what EU border agency Frontex described as tantamount to providing a "taxi" service to Europe.
But UN High Commissioner for Refugees Filippo Grandi paid tribute to the role of non-governmental organisations (NGOs), in comments about Mediterranean crossings over the weekend.
"The tireless efforts of the Italian Coast Guard, in coordination with Frontex .. and of NGOs are truly remarkable," he said in a statement.
"Together, they have saved tens of thousands of lives. In 2016, NGOs rescued more than 46,000 people in the central Mediterranean, representing over 26 percent of all rescue operations. This trend continues, reaching 33 percent since the beginning of the year."
Some 6,000 migrants were rescued over Friday and Saturday in the Mediterranean in some 40 operations coordinated from Rome by Italian coastguards, as well as by several NGOs.
But these groups, in particular SOS Mediterranee and Medecins sans Frontieres (MSF), have been for some days the target of criticism from prosecutors and politicians in Italy.
NGOs have all dismissed suggestions of de facto collusion with smugglers as a baseless slur on volunteer crews whose only mission is to save lives in the absence of EU governments acting effectively to do so.
The number of people leaving Libya in the hope of starting a new life in Europe is up nearly 50 percent this year compared with the opening months of 2016.
With most departures coming in the warm summer months, the trend points to around 250,000 people arriving over the course of 2017. Some 500,000 migrants were registered in Italy in the three years spanning 2014-16.
READ ALSO: The changing face of the Mediterranean migrant crisisPhoto: Andreas Solaro/AFP