At least eight migrants died when their boat overturned off the coast of Libya but the death toll may be much higher, the Italian coastguard said.
Four people have been rescued but the survivors reported that more than 100 were on board when the boat capsized around 50km off Libya, a coastguard spokesman said.
French and Italian naval and merchant vessels, as well as a plane and a helicopter, were involved in rescue operations, the spokesman said.
In the previous two days, coastguard and naval ships as well as privately owned fishing and merchant vessels had saved around 750 migrants from rubber and wooden boats in the central Mediterranean, but also recovered five dead bodies, the coastguard said.
The spokesman could give no details on the nationalities of those saved or those who died.
Last year, a record 181,000 boat migrants, mostly from Africa, reached Italy, according to government figures. The majority paid Libyan people traffickers to make the journey.
Last year was also the deadliest on record for migrants in the Mediterranean, with almost 5,000 deaths, according to the International Organisation for Migration.
