Around 61 migrants are believed to have drowned in the Mediterranean Sea after their vessel was swamped by high waves off the coast of Libya, the International Organization for Migration (IOM) said late on Saturday.
Most of the victims were from Nigeria, Gambia and other African countries. They included women and children.
There had been 86 people on board the boat, which departed from Zuwara on Libya's western coast, witnesses told the agency.
Authorities transferred the 25 survivors to a detention center back in Libya.
The IOM said it "provided medical support" to the survivors who the organization said were all in good condition.
'One of the world's most dangerous migration routes'
"The central Mediterranean continues to be one of the world's most dangerous migration routes," the IOM said.
IOM spokesperson Flavio Di Giacomo said that more than 2,250 people have died this year on the central Mediterranean migrant route.
He said this was a "dramatic figure which demonstrates that unfortunately not enough is being done to save lives at sea."
The Mediterranean Sea is one of the main routes for migrants in Africa to reach Europe, via Italy.
Sea migrant arrivals to Italy have almost doubled compared with the same period last year, with around 140,000 people coming ashore so far this year.
Other deadly incidents this year included the sinking of a fishing boat packed with hundreds of migrants off Greece in June.
The vessel had departed from Tobruk, Libya, and 78 deaths were recorded.
The fate of 518 remains unknown, according to an IOM report.