Raw video : More than 200 African migrants are feared dead after their boat sank off the coast of Libya, the International Organization for Migration (IOM) says.
The boat carrying around 250 people is reported to have capsized 50km (30 miles) north of the Libyan coast in stormy seas and high winds.
Libyan officials say 21 people are confirmed dead and 23 rescued.
A second boat with around 350 migrants was rescued, an official from the IOM told the Associated Press news agency.
The IOM's Laurence Hart said the rescued boat and all of its passengers were now safely back in the Libyan capital, Tripoli.
"Rescue was quick because they were near an oil platform that notified the Libyan coastal guards who quickly rescued the migrants," he said.
The missing boat is believed to be in the same area.
Coastguards are believed to be looking for two other boats, which reportedly left for Italy in recent days.
"This is the typical route for migrants from Libya to Italy," Mr Hart said.
See map of main migrant routes to Europe
Italy is to start joint sea patrols in May with Libya, aimed at stopping the heavy influx of illegal migrants.
According to IOM figures, more than 31,000 people crossed from North Africa to the Italian island of Lampedusa in 2008 alone.
The UN's refugee agency reports that two boats, carrying more than 450 people, have arrived in Italy in the past week.
Mother and son
Libyan media report that among the dead was an African woman found lifelessly clutching an infant to her bosom, says the BBC's Rana Jawad in Tripoli.
The missing vessel reportedly ran into difficulty shortly after setting off on Sunday from Sidi Bilal, near Tripoli.
"Libyan search and rescue operations led to the recovery of the bodies of those who drowned as a result of the accident, among them the bodies of 10 Egyptians," said Egyptian official Ahmed Rizk.
The nationalities of the migrants are reported to have included Egyptians, Tunisians and Palestinians.
UN High Commissioner for Refugees Antonio Guterres expressed his "great sorrow" at the loss of life.
He said it was 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".
Hundreds of migrants have died in the last few months crossing the Mediterranean Sea from North Africa to Europe, and the Gulf of Aden from Somalia to Yemen.
They are at the mercy of unscrupulous smugglers, unseaworthy vessels and the elements but many take these risks for the lure of a better life.
The smuggling season normally stops in October, and resumes again in April.
But the IOM says there has been no lull this year and the smuggling boats have been sailing right through the winter.
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