Hundreds of people are feared dead after two boats carrying up to 500 migrants capsized off the Libyan city of Zuwara, residents and officials say.
The first boat, which signalled for help early on Thursday, had nearly 50 people on board.
The second overcrowded boat, which sank much later, had about 400 passengers.
Officials said about 201 people had been rescued by the Libyan coast guard but many people appeared trapped in the hold when the boat capsized.
A detention facility for illegal migrants in Sabratha, west of Tripoli, received 147 people, an official told Reuters.
At least 100 bodies were taken to a hospital in Zuwara, west of Tripoli, a resident told the BBC.
The victims included migrants from Syria, Bangladesh and several sub-Saharan African countries, the resident said, but the information could not be independently verified.
About 2,400 migrants have died trying to cross the Mediterranean to Europe so far this year, the UN says.
More than 100,000 others have landed in Italy, whilst another 160,000 have 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 3,000 migrants saved that day.
The Swedish ship, Poseidon, docked in the port of Palermo, Sicily, on Thursday.
On Saturday, about 4,400 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, BBC North Africa correspondent Rana Jawad reports from Tunis.
Meanwhile, in Austria, police said they hoped to soon establish how many people died in a parked lorry near the Hungarian border. They suspected it could be as many as 50 people, almost certainly migrants.
The issue of the influx of migrants into Europe through land routes was raised at a summit in Vienna on Thursday.
Austrian Foreign Minister Sebastian Kurz, highlighted the need to deal with the large number of migrants heading to the EU via Western Balkan nations.
He stressed the "whole idea of the European Union without borders inside is in danger" if the bloc's external borders were not secure.
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