Medecins Sans Frontieres (MSF) said on Tuesday it fears that a boat packed with migrants sunk off the Libyan coast with more than a hundred people dead.
"We have every reason to fear the worst - that over 100 lives were lost and nobody will ever know for sure," MSF said in a statement on Twitter.
The Libyan coast guard was not immediately available for comment.
Libya is a hub for migrants and refugees, many of whom try to reach Europe in unseaworthy boats.
A boat carrying about 250 people, mainly from Eritrea and other sub-Saharan Africa and Arab countries, capsized last month off the coast near Komas, east of the capital Tripoli.
Libyan coast guards and local fishermen rescued 134 people while about 115 were missing, bringing the death toll of Mediterranean migrants to over 600 this year.
That put 2019 on course to be the sixth year in a row with more than 1,000 deaths, UNHCR spokesman Charlie Yaxley said at the time.