About 240 migrants are believed drowned in two shipwrecks off the coast of Libya, migration officials say.
The UN refugee agency was told the news by survivors brought ashore on the Italian island of Lampedusa, spokeswoman Carlotta Sami said.
No bodies have so far been recovered.
More than 4,200 migrants have died making the dangerous journey across the Mediterranean Sea this year, International Organization of Migration spokesman Leonard Doyle says.
The UN has warned 2016 could be the deadliest for migrants making the journey.
Nearly 330,000 migrants have crossed the sea so far this year, compared with more than one million in 2015.
Many of those killed in the latest two incidents are believed to be migrants from West Africa.
Ms Sami said 29 people had survived one of the shipwrecks but about 120 others were missing, feared dead. In the second, two people were picked up after swimming for safety but another 120 migrants were still missing.
Smugglers who organise the treacherous journeys overload flimsy boats and often send them off in bad weather, the UN says.
Italy has seen an increase in the trafficking of migrants from Libya ever since an EU-Turkey agreement to halt migrants travelling to the Greek islands came into force in March.
Migrant crisis: ‘Hundreds dead’ in shipwrecks off Libya