Dozens of people are missing and believed drowned after an inflatable boat carrying more than 100 people sank off the coast of Libya earlier this week, the International Organization for Migration (IOM) said Friday.

The IOM said the boat began sinking Monday just hours after leaving the North African country. Eighty people were rescued by a passing vessel and handed off to the British military ship the HMS Echo, which is taking part in the European Union’s operation to curb human trafficking in the Mediterranean.

“They don’t know how long it was before help arrived,” IOM spokesperson Flavio Di Giacomo told the French news agency AFP. He said survivors “clung to bits of the dinghy” until night fell and they were rescued.

Sixty people remain missing, the IOM said.

The Echo took part in several sea rescues this week and delivered 550 people to the Italian port city of Brindisis Friday.

Thousands of people from sub-Saharan Africa, trying to leave behind drought, poverty and war conditions, are trying to reach Europe. Nearly 100,000 have made the journey so far this year, the IOM said, and more than 10,000 this past week.

More than 2,000 people have died trying to reach Europe, the organization said Friday. Human traffickers often overload boats that are not seaworthy and push the migrants into the Mediterranean, where vessels that are part of an EU operation patrol.