Nine survivors were saved by a commercial ship and brought to the Italian island of Lampedusa on Wednesday morning, telling rescuers all other passengers had drowned.
Two of the migrants were saved from a dinghy carrying 105 people, while the other seven were among 107 people in another dinghy which set off from Libya on Saturday, UN spokesman Federico Fossi told The Local.
The sub-Saharan migrants said that all other passengers had died, while they also said another boat left Libya at the same time and has not reached Italy.
“The number of people lost at sea is probably more than 300, considering that the average number of people on board in 100,” Fossi said from Lampedusa.
The migrants said four boats left the Libyan coast, 50km from the capital Tripoli, one of which arrived in Italy earlier this week. Twenty-nine people died of hypothermia on the only one of the four boats to have reached Italy.
The migrants travelling on the missing dinghies were all from sub-Saharan countries, including Mali, Senegal and Gambia.
“Sub-Saharans that cannot pay higher fees for the journey, they are in overcrowded boats with not very powerful engines,” Fossi said.
It is not yet known whether people smugglers were on board the dinghies.
‘The level of desperation is increasing’
Oscar Spooner, spokesman for the Jesuit Refugee Service, said the deaths "could have been avoided if the resources were put in place."
He said a European search and rescue operation needed to be implemented, on the same scale of Italy's "Mare Nostrum" ("Our Sea") mission which came to an end last year.
"We believe that it's a European responsibility to protect people fleeing war and persecution, and so far with the efforts of Triton [the EU's mission] there’s too much emphasis on border control," he told The Local.
The idea that a full-scale search and rescue operation would act as a pull factor to migrants and refugees was described by Spooner as a "huge misconception". The largest group of boat migrants come from Syria, fleeing the country's ongoing civil war.
"As we’ve seen the level of desperation is increasing and people will do anything to find a safe and normal life for their families," Spooner said.