Military vessels and aircraft from a multi-nation operation are searching waters off Libya for a second day for any more survivors from an overturned boat crowded with migrants.

With seas warm and calm, rescuers have expressed hopes others might be alive on Thursday, a day after a 20-meter (66-foot) long fishing boat capsized as rescuers approached.

In the first hours after the accident, 367 survivors were rescued and 25 bodies recovered. Military officials from Ireland, which has a navy vessel in the rescue mission, said they were given an initial estimate of 600 migrants aboard the smugglers' boat. If that estimate holds, as many as 200 migrants inside the boat might have drowned.

The Irish vessel was expected to reach Sicily later Thursday with most of the survivors.