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Up to 500 migrants trying to reach Europe may have drowned off north Africa last week, the United Nations' refugee agency and an aid organization said Wednesday, although exact details of the tragedy remained unclear.

"If confirmed, as many as 500 people may have lost their lives when a large ship went down in the Mediterranean Sea at an unknown location between Libya and Italy," UNHCR said in a statement.

The International Organization for Migration (IOM) said survivors had given similar accounts to its workers and that the wreck may have "caused the deaths of upwards of 400 migrants and refugees."

It came almost exactly a year after more than 800 migrants — including children — drowned when their crowded ship capsized during an attempt to reach Italy from Libya. Only 28 people were known to have survived that incident — the worst maritime disaster the agency has ever recorded in the Mediterranean.

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A UNHCR team on Tuesday interviewed some of the 41 survivors of last week's sinking, who were rescued and taken to Kalamata in Greece, its statement added. Those rescued include 23 Somalis, 11 Ethiopians, 6 Egyptians and a Sudanese, according to the agency.

The survivors told the UNHCR they had been part of a group of between 100 and 200 who departed Tobruk, Libya last week.

After a few hours, smugglers tried to transfer the passengers onto a larger ship carrying hundreds more people in terribly overcrowded conditions, the witnesses said,told the UNHCR and the IOM.

The larger boat — now with around 500 on board, survivors told IOM — started to take on water, capsize and then sank.

"I saw my wife and my two-month old child die at sea, together with my brother-in-law," one of the survivors, Mohamed, told the IOM. "The boat was going down...down..., all the people died in a matter of minutes. After the shipwreck we were drifted at sea for a few days, without food, without anything, I [thought] I was going to die."

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The survivors told the UNHCR they had drifted at sea around three days before being spotted and rescued Saturday.

A spokesman for the Hellenic Coastguard in Greece told NBC News Wednesday that it had no information on the incident.