Up to 700 migrants - including 40 children - are feared to have died this week while crossing the Mediterranean, based on the accounts that survivors told aid workers once they reached safety.

Aid agencies UNHCR and Save the Children say that survivors' stories point to about a hundred people missing from a boat which capsized Wednesday, an incident captured by the Italian navy in a series of harrowing photographs. Another 500 appear to be missing from a boat which sank on Thursday morning, and another 100 from a third shipwreck on Friday.

According to the Italian Coast Guard, the total number of documented dead from these three shipwrecks is 65, but survivors tell a different story.

Migrants from Thursday's wreck, where over 500 are believed missing, say that they were traveling in a convoy of three boats - one rubber raft and two fishing vessels packed to the brim -- which set off from the Libyan coast early Wednesday morning. One of the fishing vessels, said to be carrying 670 people, had no motor and was being trailed by the others.

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On Thursday morning, the second fishing vessel started to take in water. Those on board tried bailing out the water, but to no avail. Eventually, the boat sank.

Italian daily La Stampa reported that a Nigerian female survivor told aid workers: "We tried everything to stop the water, to bail it out of the boat. We used our hands, plastic glasses. For two hours we fought against the water but it was useless. It began to flood the boat, and those below deck had no chance. Woman, men, children, many children, were trapped, and drowned."

As the boat began to sink, the smuggler believed to be in command ordered the rope linking the two vessels to be cut. Survivors say that the rope snapped back and decapitated a woman. The alleged smuggler -- a 28-year-old Sudanese -- has since been arrested by Italian police.

The Italian Coast Guard says it coordinated the rescue of approximately 14,000 people in the Mediterranean this past week, on 70 rubber rafts and 10 wooden vessels which departed from Libya. According to the International Organization for Migration (IOM,) over 200,000 people have made the Mediterranean crossing so far this year, about 40,000 of them into Italy.

Before Sunday's announcement that 700 migrants may have died this week alone, the IOM estimated the number of dead so far this year at 1,260. The IOM says 4,000 migrants died in the Mediterranean in 2015.