Hundreds of migrants attempting the perilous journey across the Mediterranean are feared to have drowned after their overcrowded fishing boat capsized off Libya, Italy's coastguard said Wednesday.

The boat, which had run into difficulty and sent out a distress call, overturned after the first rescue vessel came in sight, as migrants rushed to one side. Some 100 survivors were plucked from the water and the search for others is continuing, the coast guard said.