Migrant boat capsizes in Mediterranean, at least 25 dead
UNHCR said about 400 people had been rescued and 25 bodies had been recovered so far.
Officials said it was still not clear how many people were on board. The Italian Coast Guard and the United Nations refugee agency UNHCR said about 400 people had been rescued and 25 bodies had been recovered so far.
The operation, which included vessels from the Italian and Irish navies and the humanitarian group Medecins sans Frontieres (MSF) was continuing.
MSF, whose rescue ship Dignity 1 was in the area, said in a tweet that the boat was believed to have been carrying about 700 people.
"The team on the Dignity 1 can confirm that there have tragically been many deaths but does not have figures at this stage," one of its tweets said.
Irish Defence Minister Simon Coveney said in a statement he feared that "loss of life is likely to be significant".
UNHCR spokesman Federico Fossi said as many as 100 people may have been in the hull when the boat capsized as rescue ships approached it.
The Mediterranean Sea is the world's most deadly border area for migrants. More than 2,000 migrants and refugees have died so far this year in attempts to reach Europe by boat, compared with 3,279 deaths during the whole of last year, the International Organization for Migration said on Tuesday.
In April, a fishing boat with up to 800 migrants sank, making it the deadliest shipwreck in the Mediterranean for decades and a symbol of Europe's long-running migrant crisis.
The 20-metre vessel capsized as it approached a merchant ship that had come to its assistance.