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, 75 miles (120 kilometers) 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.

Libya has become one of the main departure points for migrants from sub-Saharan Africa trying to reach Europe, with most heading for the Italian coast around 190 miles (300 kilometers) away.

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.