More than 200 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.
Twelve bodies have 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 a dinghy - which was reportedly carrying about 140 people including six children and about 20 women, some of them pregnant - capsized 25 miles (40km) off the Libyan coast. Twenty-nine people were rescued, she said, and 12 bodies were recovered.
In a separate rescue operation, two women found swimming at sea told rescuers that 128 other people had died in their wreck.
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.