"We confirm from our independent sources of information that the number of missing people is hitting 10,000 persons so far," Tamer Ramadan, head of the ICRC mission in Libya, said.
"The death toll is huge and might reach thousands," he told reporters in Geneva via video link from Tunis.
More than 3,000 people have been reported dead and thousands remain unaccounted for after floods caused by Storm Daniel struck eastern Libya, according to Libyan officials.
Initial reports indicate that dozens of villages and towns were submerged by floods amid widespread damage to infrastructure.
Local authorities in Derna said two dams have collapsed in the city, adding to the devastating floods.
On Monday, Libya’s Presidency Council appealed to friendly countries and international aid groups to provide aid to the flood-stricken areas in the eastern region.