The Libyan Red Crescent, which was cited in the report, later denied the toll, saying it was “shocked” to see its “name mixed up with these figures”.
People whose homes were swept away by flooding in Libya's eastern city of Derna a week ago faced the dilemma on Sunday of whether to stay and risk infection or flee through areas where landmines have been displaced by the torrents.
Thousands of people were killed after two dams above Derna broke on Sept. 10 during a powerful storm, bringing down residential blocks lining a usually dry river bed as people slept. Many bodies have been washed out to sea.
The U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA) said approximately 11,300 people had died - more than double the figure given by the International Organization for Migration on Friday.
The Libyan Red Crescent, which OCHA cited for the data, distanced itself from the report but OCHA stood by it.
More than 1,000 have already been buried in mass graves, according to the United Nations, and aid groups have warned against the practice. Libyan authorities have confirmed that 150 people have been poisoned by polluted water in the flood-hit areas.