An Italian navy captain who led the rescue of hundreds of people from a capsized fishing boat off Libya described Thursday how one of his men had pulled a drowning migrant from the waves by his hair.

And Francesco Iavazzo, the skipper of the Bettica patrol boat, said he was not in the least surprised to see several of his crew throw themselves into the Mediterranean to clutch floundering survivors from the jaws of death at the height of Wednesday's drama.

"When you work for the military, the navy particularly, you do what you have to do, you don't do it thinking about what's in your job description," he said in one of several satellite phone interviews with Italian media.

"At sea, the saving of human lives is something sacred."

Iavazzo's crew did surprise the skipper however after his boat had pulled more than 400 people out of the water, including 43 children, 10 of whom were babies.

"In this tragedy the one small consolation is that we saved all the children," he said.

"And they all got toy bears that my sailors produced from goodness knows where. I saw a baby girl clutching a white one that was bigger than her."

In total, the Bettica and its navy sister ship the Bergamini rescued 562 people from the capsized fishing boat.

Sailors also recovered five corpses and Iavazzo said it was possible others had been unable to get off the boat and had perished when it sank.

- Fatal panic -

The Bettica was headed on Thursday for Sicily, where the survivors will be added to the list of nearly 40,000 migrants to arrive at Italy's southern ports this year.

Salvatore Vitiello, the admiral who heads Italy's "Safe Seas" operation to protect merchant shipping off Libya, was on board the Bergamini during the rescue.

He described how the capsize happened after nearly half the passengers had been transferred to safety.

"At that point there were still dozens of people below deck and in their desperation to get out and be saved they all moved to one side of the hull, causing the boat to roll over.

"But by using dinghies and rafts we were able to get nearly all of them out of the water alive."

Capsizes have been responsible for some of the deadliest sinkings of Europe's migrant crisis and often panic is the fatal factor.

- 'Freedom, freedom!' -

Antoine Laurent, a French merchant navy officer who is currently on board the migrant rescue boat Aquarius, said people traffickers' boats were often inherently unstable.

"On normal boats there is a lot of weight at the bottom -- fuel, equipment etc -- which ensures the centre of gravity is in the middle of the hull.

"On migrant boats it is people below deck who provide the ballast and often they end up trying to get out as quickly as possible because the heat is unbearable or there is gas leaking somewhere.

"It is very hard to stay calm during transfers, even if the boat is stable and the sea is calm the migrants are always on the brink of panic.

"Most of them do not know how to swim -- to them the water is like a torrent of lava, for us, if you fall in you're dead."

Iavazzo said the survivors had mostly been picked up by dinghies fitted with hydrojet motors to avoid the risk of propellers injuring people in the water.

"As soon as people come on board we warm them up in the classic way with hot tea and blankets and the medical team screen them for emergency cases that might require helicopter evacuation."

Vitiello, who has now been at sea for three and a half months, admitted he had been deeply moved by the reaction of the survivors who ended up on his deck.

"It was a beautiful thing, seeing all these young people crying with happiness, hugging our sailors and shouting 'Freedom, Freedom!'."