Up to 30 migrants are feared dead after a boat capsized in the Mediterranean off the Libyan coast while some 77 were rescued by EU naval units from the sea.
Ships from an EU task force and Italy's coast guard raced to the scene 35 nautical miles (65km) off the coast as survivors clung to the hull or swam.
The Italian navy rescued 562 migrants from a capsized boat on Wednesday.
It has now emerged that the death toll from that shipwreck may have been as great as 100.
Five deaths were reported by the Italian coast guard at the time but the International Organization of Migration (IOM) believes others were trapped in the upturned hull.
Aid agencies say the crossing between Libya and Italy is the main route for migrants since an EU deal with Turkey curbed the number sailing across the Aegean to Greece.
In other developments:
Some 20 people living in the "Jungle" migrant camp in Calais, France, were injured in a meal-time brawl involving around 200 Afghans and Sudanese, with one person wounded by gunfire and others by knives and sticks, police say
The mayor and 11 councillors in the northern Italian border town of Ventimiglia suspended their membership of the ruling Democratic Party. They stood down over what they said was the government's failure to take action over the plight of about 150 migrants camped out in unhygienic conditions along a riverbank
Greek police announced that the evacuation of the Idomeni migrant camp on the closed border with Macedonia was complete
A Luxembourg reconnaissance plane spotted the capsized boat on Thursday and raised the alert.
"We estimate the dead to be between 20 and 30 people," Captain Antonello de Renzis Sonnino, spokesman for the EU Navfor Med operation to combat people smugglers in the region, told AFP news agency, as the rescue continued.
The Spanish frigate Reina Sofia, backed by the Italian coastguard, threw life floats and jackets to those in the water.
A second, Spanish plane was also involved in the rescue operation, Italy's Ansa news agency reports (in Italian).
In all, the Italian coast guard rescued 2,600 migrants on Thursday alone, it said in a tweet (in Italian).
Survivors of Wednesday's incident were brought ashore in Porto Empedocle, on the Italian island of Sicily, where IOM teams interviewed them
"The survivors who talked to our teams when they landed at Porto Empedocle spoke of 100 missing people who were stuck in the hull," IOM spokesman Flavio Di Giacomo told AFP news agency.
Horrific scenes of people clinging desperately to the upturned boat or dropping into the sea were caught on camera.
Mr Di Giacomo added that most of the 562 survivors were Moroccans but there were also "many Tunisians and two Syrian families".
Speaking to BBC News, he said: "According to stories gathered by IOM staff, those are people who were living in Libya and who are fleeing the country which is now too dangerous."