Eighteen migrants have drowned after their boat sank off Libya, the coastguard said Tuesday, the latest tragedy on the dangerous Mediterranean Sea crossing to Europe.

Rescue vessels picked up 51 survivors from Sunday night's shipwreck, a coastguard official in the port of Zuwara, 120 kilometres (75 miles) west of the capital Tripoli, told AFP.

The International Organization for Migration had earlier reported 16 migrants missing, including a woman and a child.

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.

Nearly 970 migrants have died trying to reach Europe from Libya since the start of the year.

Last month, the IOM said the number of people who had died trying to cross the Mediterranean nearly doubled in the first half of 2021 compared with the same period last year.

Despite persistent violence since the 2011 overthrow of longtime dictator Moamer Kadhafi, Libya has become one of the main departure points for migrants from sub-Saharan Africa trying to reach Europe.

Most head for the Italian coast around 300 kilometres (190 miles) away.

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.

The Libyan coastguard picked up more than 13,000 people in the first half of this year, exceeding the total figure for 2020, according to the UN High Commissioner for Refugees.

That figure has now risen to 20,257, according to the IOM.