Seven members of the Libyan National Army were missing, LNA spokesman Ahmad Mesmari said.
Footage on social media showed people stranded on the roofs of their vehicles while trying get help in heavy floods as Storm Daniel hit the cities of Benghazi, Sousse, Al Bayda, Al-Marj and Derna on Sunday and Monday.
"We were asleep, and when we woke up, we found water besieging the house. We are inside and trying to get out," Derna resident Ahmed Mohamed told Reuters by phone on Monday.
Search and rescue operations were ongoing, witnesses said. Authorities declared a state of extreme emergency, closing schools and stores and imposing a curfew.
Four major oil ports in Libya, Ras Lanuf, Zueitina, Brega and Es Sidra, were closed from Saturday evening for three days, two oil engineers told Reuters.
The prime minister of the interim government in Tripoli, Abdulhamid Dbeiba, said on Sunday he had directed all state agencies to "immediately deal" with the damage and floods in eastern cities.
The United Nations in Libya said it was following the storm closely and would "provide urgent relief assistance in support of response efforts at local and national levels".
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