Officials say 82 bodies have been recovered after a boat full of refugees capsized near the Libyan coast.
A Red Crescent official said 198 people had been rescued but another 100 were still missing.
The boat was thought to be carrying 400 people trying to reach Europe.
Officials said many people appeared to have been trapped in the hold when the boat capsized near the western Libyan town of Zuwara.
A spokesperson for the UNHCR, William Spindler, said the coastguard was still bringing survivors to shore.
A detention facility for migrants in Sabratha, west of Tripoli, received 147 people, an official told Reuters.
The victims included migrants from Syria, Bangladesh and several sub-Saharan African countries, the resident said, but the information could not be independently verified.
The United Nations commission on refugees (UNHCR) said the number of refugees and migrants crossing the Mediterranean has passed 300,000 already this year, up from 219,000 in the whole of 2014.
About 2400 people had died trying to cross the Mediterranean to Europe so far this year, the UN said.
More than 100,000 others had landed in Italy, whilst another 160,000 had crossed to Greece.
On Wednesday, the bodies of at least 51 people were found in the hold of a stricken ship off Libya's coast.
They were picked up by a Swedish coastguard ship that also rescued more than 400 survivors - among at least 3000 migrants saved that day.
The Swedish ship, Poseidon, docked in the port of Palermo, Sicily, on Thursday.
On Saturday, about 4400 migrants were rescued from boats off the coast of Libya, in one of the biggest single-day operations mounted to date.
Many of those who attempt the journey are fleeing conflict or persecution, and set off from Libya in unseaworthy boats organised by smugglers. Libya has had two competing governments for the past year and is largely ruled by rival militias. The Libyan coastguard has limited capacity to undertake large-scale rescue missions at sea.