The Irish navy said "significant loss of life" was feared and Medecins Sans Frontieres confirmed "many deaths"
Rescuers are searching for hundreds of migrants in the Mediterranean after a boat carrying as many as 600 people capsized off the coast of Libya.
The Irish navy said "significant loss of life" was feared and Medecins Sans Frontieres confirmed "many deaths".
But the Italian coast guard and the UN refugee agency say about 400 have been rescued and 25 bodies recovered so far.
More than 2,000 migrants are said to have died in 2015 trying to cross the Mediterranean to reach Europe.
Wednesday's incident occurred when the packed fishing boat ran into rough weather about 15 miles (25km) from the Libya's coast.
A distress call was picked up in Sicily and one of the first ships on the scene was an Irish navy vessel. But as she launched her boats, the migrants apparently moved to one side of the fishing boat, causing it to capsize.
At least three other ships and three helicopters were reported to have arrived in the area shortly afterwards.
'Capsized very fast'
Cmdr Filippo Marini of the Italian coast guard said the rescue operation was still ongoing and now involved seven ships, .
He said survivors had indicated that between 400 and 600 people were on board the boat when it capsized.
Melissa Fleming, a UN refugee agency (UNHCR) spokeswoman, said about 100 people are thought to have been travelling in the boat's hull at the time, adding: "The boat capsized very fast".
The Italian coast guard rescued about 300 migrants adrift in the Mediterranean on Monday
Medecins Sans Frontieres (MSF) said its Dignity I vessel was at the scene and two more of its boats were on their way to the area.
"The team on the #Dignity1 can confirm that there have tragically been many deaths but does not have figures at this stage," MSF said in a tweet.
It said the same vessel had also been involved in another rescue operation earlier on Wednesday, in which about 100 migrants were rescued.
The Mediterranean Sea is the world's most deadly border area for migrants.
In April, a fishing boat carrying about 800 migrants sank off the coast of Libya in what the UN called the deadliest incident in the Mediterranean ever recorded.
Earlier this week, the IOM warned that the number of migrants attempting to cross the sea is much higher than in the same period last year.
"It is unacceptable that in the 21st Century people fleeing from conflict, persecutions, misery and land degradation must endure such terrible experiences [...] and then die on Europe's doorstep," IOM director general William Lacy Swing said in a statement.