Updated 7am

At least 65 migrants have died after their boat capsized in the Mediterranean off the coast of Tunisia, the UN refugee agency said.

Sixteen people were rescued between Tunisia and Malta, UNHCR said in a statement.

The bodies of three people who drowned in the incident were found Friday, a spokesman for the Tunisian defence ministry said, adding the boat left Zuwara on the northwestern Libyan coast on Thursday.

A fishing boat picked up 16 survivors, who were transferred on board one of three military vessels involved in the search and rescue operation, said spokesman Mohamed Zekri.

A Maltese helicopter was also mobilised, he said.

According to those rescued, between 60 to 70 people from sub-Saharan Africa were thought to be on board, Zekri added.

Interior ministry spokesman Sofiene Zaag said, however, around 75 people were believed to have been on board the Italy-bound boat when it left Libya.

The Red Crescent said there could have been as many as 90 passengers on board the ill-fated vessel, which could indicate a much steeper death toll.

"We will probably never know the exact number of those who died," said Mongi Slim, a Red Crescent official in the southern Tunisian town of Zarzis, where those rescued were taken.

Italy's far-right Interior Minister Matteo Salvini has vowed to close the country's ports to migrants, although earlier Friday dozens who had been rescued at sea disembarked in Sicily.

Rome's populist government has taken an increasingly hard line on migration, and Salvini, head of the anti-immigrant League party, last month signed a new directive banning charity vessels from rescuing migrants off Libya.

'World's deadliest sea crossing'

The UN agency for refugees UNHCR meanwhile called for stepped up search and rescue operations to avoid future tragedies in the Mediterranean, which it calls the "world's deadliest sea crossing".

"Across the region we need to strengthen the capacity of search and rescue operations," said Vincent Cochetel, the agency's special envoy for the Mediterranean.

"If we don't act now, we're almost certain to see more tragic events in the coming weeks and months," he warned.

According to the UNHCR, the journey across the Mediterranean "is becoming increasingly fatal for those who risk it".

"In the first four months of this year, one person has died (crossing the Mediterranean) for every three that have reached European shores, after departing from Libya," it said.

That is up from last year's toll of one person in 14, according to the UN refugee agency.

Libya, which has been wracked by chaos since the 2011 uprising that killed veteran dictator Moamer Kadhafi, has long been a major transit route for people desperate to reach Europe.

Over the past week, and despite a surge in violence following the launch of an assault on Tripoli by strongman Khalifa Haftar, Libyan authorities have rescued hundreds of migrants stranded at sea.

According to the International Organization for Migration, 101 migrants were returned to Libya on Friday, two days after Libyan coastguard intercepted boats.

The UN and rights groups have repeatedly decried the conditions in which migrants are held in Libya, where around 6,000 migrants are at detention centres, according to the IOM.

The Tunisian Forum for Economic and Social Rights NGO has denounced what it said were the European Union's "restrictive and inhumane policies" which it said led to the latest "human tragedy".

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