Migrant rescue charities operating in the Mediterranean said Tuesday that an Italian boat may have violated international law by taking more than 100 migrants it had saved back to Libya.
The Asso Ventotto, an Italian offshore supply ship, picked up 108 migrants in international waters late Monday.
They were found on a dinghy around 60 nautical miles northwest of Libya's capital Tripoli, according to a maritime traffic site. 
Local media reported that Italy's coastguard instructed the Asso Ventotto to coordinate the rescue operation with the Libyan authorities. 
The boat then transported those aboard to a port in Tripoli. 
The UN High Commissioner for Refugees (UNHCR) in Italy tweeted: "Libya is not a safe port and this could lead to a violation of international law," adding that they were collecting "all the necessary information on the case".
German NGO Sea-Watch also condemned "the first pushback by an Italian vessel for years," on Twitter.
In 2012, the European Court of Human Rights ruled that Italy violated human rights by sending migrants intercepted at sea back to Libya in 2009.
The court said the practice violated international obligations to not return individuals to countries where they could be at risk of human rights abuses.
Many migrants trying to reach Europe are desperate not to go back to Libya, where they have been reported to face abuse and rape in detention centres. 
Up until recently, the Italian coastguard led rescue missions and the migrants were taken to Italy.
However Italy's new populist government, in power since June, has taken a hardline stance on immigration and ordered the coastguard to let Libyan authorities take charge of such operations.
As NGOs expressed their dismay over the latest boat controversy, Italy's far-right interior minister Matteo Salvini, who has closed the country's ports to migrant rescue ships, praised the Libyan authorities.
"Over the last hours the Libyan coastguard has saved and brought back 611 immigrants to Libya," he tweeted.
"NGOs protest and traffickers lose their business? Well, we are going on like this!" he said with his signature hashtags "#Closedports, #Openhearts".
However the speaker of Italy's lower house, Roberto Fico, who belongs to the Five Star Movement that governs in a coalition with Salvini's League, appeared to disagree with sending migrants to Libya.
"Libya is not a safe place... it is clear that you cannot leave migrants there," Fico said as he met with protesters denouncing the sale of Italian boats to Libya's coastguard on Monday.
Several commercial ships that have tried to take rescued migrants to Italy -- as was standard procedure under the former centre-left government -- have found themselves blocked by Salvini's policy and stranded for days at sea searching for a port where they can disembark. 
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