"My colleagues are interviewing the survivors... Who arrived this afternoon in Augusta (Sicily), and they are talking of 35 to 40 people missing at sea," UNHCR's spokesman in Italy, Federico Fossi, told AFP of the accident on Wednesday.
The refugee agency said most of the migrants were men from sub-Saharan African countries, including Somalia, Eritrea, Benin and Mali.
Italian daily La Reppublica said three dinghies set sail from Tripoli but one of them, with 120 people on board, started filling up with water and eventually sank.
One 22-year-old Ghanaian cited by the newspaper claimed that some 30 of his fellow travellers had died.
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International NGO Save the Children said women and children were among those missing.
"We have spoken to several (survivors) and their versions match. I have in front of me a young boy in tears, who says he has lost his brother. The victims are all from sub-Saharan Africa," Save the Children spokeswoman Giovanna di Benedetto was quoted as saying by La Reppublica.
The migrants are seeking refuge from conflicts and poverty in Africa and the Middle East.