POZZALLO, Sicily » Over 700 migrants are feared dead in three Mediterranean Sea shipwrecks south of Italy in the last few days as they tried desperately to reach Europe in unseaworthy smuggling boats, the U.N. refugee agency said Sunday.

The shipwrecks over three days appear to account for the largest loss of life reported in the Mediterranean since April 2015, when a single ship sank with an estimated 800 people trapped inside.

Carlotta Sami, spokeswoman for UNHCR, told The Associated Press by phone that an estimated 100 people are missing from a smugglers’ boat that capsized Wednesday off the coast of Libya. The Italian navy took horrific pictures of that capsizing even as it rushed to rescue as many people as possible from the sea.

Sami said about 550 other migrants and refugees are missing from a smuggling boat that capsized Thursday morning after leaving the western Libyan port of Sabratha a day earlier.

Refugees who saw that boat sink told her agency it was carrying about 670 people, didn’t have an engine and was being towed by another packed smuggling boat before it capsized. She said about 25 people from the capsized boat managed to reach the first boat and survive, 79 others were rescued by international patrol boats and 15 bodies were recovered.

Italian police said survivors identified the commander of the boat with the working engine as a 28-year-old Sudanese man, who has been arrested.

In a third shipwreck on Friday, Sami said 135 people were rescued, 45 bodies were recovered and an unknown numbers of migrants were still missing.

Because the bodies went missing in the open sea, it is impossible to verify the numbers who died. Humanitarian organizations and rescue authorities typically rely on survivors’ accounts to piece together what happened.

Italian police corroborated the UNHCR description of Thursday’s sinking in their own interviews with survivors, but came up with different numbers of possible missing.

They say, according to survivors, the boat being towed was carrying about 500 migrants when it starting taking on water after about eight hours at sea. Efforts to empty the water — with a line of migrants passing a few 5-liter bailing cans — were insufficient and the boat was completely under water after an hour and a half, police said. At that point, the commander of the first smuggling boat doing the towing ordered the tow rope to be cut.

The migrants on the top deck of the sinking boat jumped into the sea, while those below deck, estimated at 300, sank with the ship, police said. Of those who jumped into the water, just 90 were rescued.

Survivors were taken to the Italian ports of Taranto on the mainland and Pozzallo on the island of Sicily. Sami says the U.N. agency is trying to gather information with sensitivity considering that most of the new arrivals are either shipwreck survivors or traumatized by what they saw.

Italy’s southern islands are the main destinations for countless numbers of smuggling boats launched from the shores of lawless Libya each week packed with people seeking jobs and safety in Europe. Hundreds of migrants drown each year attempting the dangerous Mediterranean Sea crossing.

Warmer waters and calmer weather of late have only increased the migrants’ attempts to reach Europe. Last week, over 4,000 migrants were rescued at sea in one day alone by an Italian-led naval operation.

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Colleen Barry in Milan contributed to this report.