Search teams in the Mediterranean say they do not expect to find any more survivors from a boat carrying around 600 migrants which sank off Libya.
Officials initially feared hundreds had drowned but the UN refugee agency (UNHCR) said 400 people were rescued.
The Italian coastguard said 25 bodies have been recovered so far but it is unclear how many people are missing.
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 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, the LÉ Niamh. But as she launched her boats, the migrants apparently moved to one side of the fishing boat, causing it to capsize.
"Those that were on deck would have managed to jump, some of them drowned and some of them were saved," said Martin Xuereb of the Migrant Offshore Aid Station in Malta, one the groups involved in the rescue effort.
"I think it's unlikely that any additional survivors will be picked up."
Seven ships as well as helicopters have been involved in the search operation.
'Capsized very fast'
Cmdr Filippo Marini of the Italian coast guard 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".
Medecins Sans Frontieres (MSF) said its Dignity I vessel was one of the first to arrive at the scene. Two more of the medical charity's boats arrived later in the day.
"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.
'Lack of resources'
"It was a horrific sight, people desperately clinging to lifebelts, boats and anything they could to fight for their lives," Juan Matias, an MSF project co-ordinator on the Dignity I, said in a statement.
MSF said the vessel was initially diverted from heading to the fishing boat in order to deal with a separate vessel that was in trouble. The group said it rescued nearly 100 migrants in that incident before answering the fishing boat's distress call.
"The fact that we were first called to assist this boat and then shortly afterwards sent to another one highlights the severe lack of resources available for rescue operations," Mr Matias said.
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.
EU leaders agreed to increase spending on its search and rescue operation in the region after that incident but smugglers continue to send desperate migrants on the dangerous journey.
Earlier this week, the IOM warned that the number of migrants attempting to make the crossing 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.