More than 50 people went missing and 33 were rescued, mostly from Bangladesh after their boat headed from Libya to Europe sank off Tunisia, the defence ministry said Tuesday.

Defence ministry spokesman Mohamed Zikri said the survivors of the shipwreck were picked up after clinging to an oil platform off the southern coast of Tunisia.

"There are 33 survivors, all apparently from Bangladesh," Flavio Di Giacomo, a spokesman for the International Organization for Migration, told AFP.

"At least 50 are missing."

The boat, crammed with over 90 passengers, left the Libyan port of Zuwara on Sunday.

It was not immediately clear what caused the boat to sink, but vessels leaving the North African coast for Europe are often heavily overloaded makeshift crafts, departing at night even in rough weather to avoid detection by the coastguard.

Tunisian rescuers were bringing the survivors to the port of Zarzis, some 100 kilometres (70 miles) northwest of Zuwara.

"We don't know the nationality of the more than 50 who are missing," Di Giacomo added.

Departures from Libya, a key gateway for Europe-bound migrants, have increased since the start of the year.

Neighbouring Tunisia regularly provides assistance to migrants in difficulty in the central Mediterranean, one of the deadliest migration routes, according to the United Nations.

The UN says more than 700 people have died trying to cross the Mediterranean this year, compared to 1,400 for the whole of last year.

Reconsider' support 

On Monday, the Tunisian navy said it had rescued more than 100 migrants, mainly from Bangladesh and Sudan, whose boat was "on the verge of sinking".

That boat reportedly also departed Sunday from Zuwara.

Several boats were also stopped by Libyan coastguards and brought back to shore overnight Sunday to Monday.

"Two days ago about 680 migrants were intercepted at sea and returned to Libya," Di Giacomo said.

"Almost 9,000 people have been intercepted at sea and returned to Libya in 2021 so far," he added.

Safa Msehli, IOM spokeswoman for the Geneva-based UN agency, said that support for search and rescue teams should depend on "no one being arbitrarily detained or subjected to human rights violations".

The European Union has for several years supported Libyan forces to try to stem migration, despite often grim conditions in detention centres in Libya.

International agencies have repeatedly denounced the return to Libya of migrants intercepted at sea.

In Lisbon last week, Tunisia's Prime Minister Hichem Mechichi repeated his country's opposition to setting up migrant reception centres on Tunisian soil, amid attempts to find safe places for asylum seekers.

Italy's interior minister is expected in Tunisia later this week, together with the EU commissioner for home affairs, Ylva Johansson, to discuss assistance and repatriations.

As at May 16, over 13,000 people had arrived in Italy illegally by sea this year, double the number over the same period last year, according to the UN. It says almost 9,000 of them departed from Libya.

According to the UN refugee agency, migrants from Bangladesh, Sudan and Guinea made up the largest groups of arrivals in the first three months of this year.