Up to 700 migrants are feared drowned in a series of shipwrecks off the coast of Libya in the past few days, the UN refugee agency says.
The boats sank south of Italy on Wednesday, Thursday and Friday as the migrants tried to reach Europe in unseaworthy vessels.
Spring weather has led to a surge of people attempting the perilous crossing from Africa to Europe.
It is now a key migration route since a deal curbed numbers sailing to Greece.
Carlotta Sami, spokeswoman for UNHCR, said almost 100 migrants were missing from a smugglers' boat which capsized on Wednesday.
About 550 other migrants are missing from a boat which overturned on Thursday morning after leaving the Libyan port of Sabratha on Wednesday. Survivors said the boat had no engine and was being towed by a second smuggling vessel.
In a third shipwreck on Friday, 135 people were rescued, 45 bodies pulled from the water and an unspecified number of others are missing.
Meanwhile, the MSF Sea group suggested the death toll from the last week could be as high as 900.
Survivors are being taken to the Italian ports of Taranto and Pozzallo.
Italian authorities have said the rescue of more than 600 migrants off Libya on Saturday by a flotilla of EU ships took the weekly total to at least 13,000.
The rescues were the latest by a multinational patrol of ships operating in the Mediterranean.