A United Nations humanitarian office report Tuesday said 400 migrants were killed in the floods that hit eastern Libya last week, citing hospital reports.
Thousands of people were declared missing after the floods triggered by hurricane-strength Storm Daniel hit the war-scarred North African country on September 10.
The report quoted the World Health Organisation as saying that 4,000 deaths had so far been reported in Libya by hospitals, including 400 migrants.
Thousands of African and Middle Eastern migrants are temporarily based in Libya and many make the perilous Mediterranean crossing each year to flee poverty and conflict.
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The International Organization for Migration had previously said that over 100,000 migrants lived in flood-hit areas, including more than 8,000 in the city of Derna. They are mostly from Chad, Egypt, and Sudan, it said.
Libya is a major gateway for migrants and asylum seekers attempting perilous sea voyages in often rickety boats in the hope of a better life in Europe.
An estimated 600,000 migrants live in the war-scarred country, which has seen 12 years of stop-start conflict since the 2011 NATO-backed revolt that toppled strongman Muammar Qaddafi.
It is not yet clear as to if Nigerian migrants in the country were among the flood casualties or missing persons.
Libyan authorities had on August 21 repatriated 161 Nigerians, as part of a UN-backed voluntary return scheme as some North African countries see a spike in irregular migration, officials said.