Up to three boats capsized off the coast of Libya, having set sail with hundreds of migrants in the direction of Italy over the past few days, many of whom are missing and feared dead, humanitarian officials said on Tuesday.
Officials with the United Nations' refugee agency UNHCR and the International Organisation for Migration (IOM) said these boats tend to be generally severely overcrowded.
"There is no safety equipment on board, they are overloaded," said Jean-Phillipe Chauzy with the IOM, who estimated that at least 300 people were missing, though the number could reach 500.
The boat operators aimed to "cram as many people in as possible" with "no respect for the safety and dignity of these people", Chauzy told reporters in Geneva.
Reports from Libya early in the morning said 21 bodies and 23 survivors had been recovered from the Mediterranean sea. One boat was carrying a reported 257 people and another had 340 on board.
There were also reports of a fourth boat which was turned back to safety, the IOM said.
"This is the beginning of the smuggling season in the Mediterranean," UNHCR spokesman Ron Redmon said, noting that the "tragic incident" highlighted the dangers facing so-called irregular migrants.
The UN's High Commissioner for refugees, Antonio Guterres, was quoted as saying that this is the "latest tragic example of a global phenomenon in which desperate people take desperate measures to escape conflict, persecution and poverty in search of a better life".
The mixed movement of economic migrants and asylum seekers was making distinguishing between them more difficult.
Redmond said the rescued people had been taken to detention centres in Libya and that UNHCR had been asked to give humanitarian assistance to those in need and screen for refugees.
Libya, with a relatively well-off economy based on oil and natural gas reserves, has become a destination for migrants from other parts of Africa, particularly from the western coast.
The IOM estimated some 1 million to 1.5 million undocumented migrants worked in Libya in the informal sector.
Last year, UNHCR said, nearly 37,000 people arrived in Italy by way of the sea from North Africa. So far this week, over 450 migrants landed in Sicily and Lampedusa.