Up to 700 migrants are feared to have drowned in deadly shipwrecks off the coast of Libya this week, the UN's refugee agency said Sunday, citing survivor testimony.

"The situation is chaotic, we cannot be sure of the numbers, but we fear up to 700 people may have drowned in three shipwrecks this week," one of which was believed to have claimed over 500 lives, UNHCR spokesman Federico Fossi told AFP.

About 500 migrants were declared lost after the fishing boat they were on capsized on Thursday morning off Libya, with survivors saying at least 40 of the dead were children, including newborn babies.

Some 100 people were missing after a boat sank Wednesday and 45 bodies were recovered from a wreck that happened Friday, with more feared lost.

"We'll never know the exact number, we'll never know their identity, but survivors tell that over 500 human beings died" in Thursday's shipwreck, Carlotta Sami, UNHCR spokeswoman, said on Twitter.

Brought to safety at the Italian ports of Taranto and Pozzallo, survivors told the UNHCR and Save the Children how their boat had sunk after a high-seas drama which saw one woman decapitated.

Giovanna Di Benedetto, Save the Children's spokesperson in Sicily, told AFP it was impossible to verify the numbers involved but survivors spoke of around 1,100 people setting out from Sabratha in Libya on Wednesday in two fishing boats and a dinghy.

"The first boat, carrying some 500 people, was reportedly towing the second, which was carrying another 500. But the second boat began to sink. Some people tried to swim to the first boat, others held onto the rope linking the vessels," she said.

According to the survivors, the first boat's Sudanese captain cut the rope, which snapped back and decapitated a woman.

The second boat quickly sank, taking those packed tightly into the hold down with it.

The Sudanese was arrested on his arrival in Pozzallo along with three other suspected people traffickers, Italian media reports said.

Source: AFP