At least 700 migrants may have died at sea over the past week in the busiest week of migrant crossings from Libya towards Italy this year, Medecins San Frontieres and the UN refugee agency said.
About 14,000 have been rescued since last Monday amid calm seas, and there have been at least three confirmed instances of boats sinking, Reuters said.
But the number of dead can only be estimated based on survivor testimony, which is still being collected, the news agency said.
“We will never know exact numbers,” Medecins San Frontieres said in a Tweet on Sunday after estimating that 900 had died during the week.
The United Nations High Commissioner for Refugees (UNHCR) said more than 700 had drowned.
Migrants interviewed on Saturday in the Sicilian port of Pozzallo told of a large fishing boat that overturned and sank on Thursday with many women and children on board.
Initial estimates were that 400 people died, but the UN Refugee agency said on Sunday there may have been about 670 passengers on board.
According to testimony collected by EU border agency Frontex, when the motorless fishing boat capsized, 25 swam to the boat that had been towing it, while 79 to 89 others were saved by rescuers and 15 bodies were recovered.
This meant more than 550 died, the UNHCR said.
The migrants – fleeing wars, oppression and poverty – often do not know how to swim and do not have life jackets.
They pay hundreds or thousands of dollars to make the crossing from Libya to Italy, by far the most dangerous border passage for migrants in the world.
Last week’s arrivals included Eritreans, Sudanese, Nigerians and many other West Africans, humanitarian groups say.
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