TUNIS: More than 50 people went missing after a migrant boat headed from Libya to Europe sank off Tunisia, the defence ministry said Tuesday.

Defence ministry spokesman Mohamed Zikri said 32 survivors of the shipwreck were picked up after clinging to an oil platform off the southern coast of Tunisia.

All of those rescued are from Bangladesh, he added.

Flavio Di Giacomo, a spokesman for the International Organization for Migration, told AFP, "We don't know the nationality of the more than 50 who are missing".

The boat, crammed with around 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.

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

There were 11,000 departures between January and April, 73 percent more than the same period last year, due in particular to the "deterioration" of the situation for foreigners in the country, according to the UN refugee agency, UNHCR.

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