Opposition forces are battling troops loyal to Muammar Gaddafi in the western city of Zawiya.
It is the first major fighting in the oil port since government troops crushed opposition forces there in March.
Guma el-Gamaty, a London-based spokesman for the rebels' national council, said the opposition fighters are in control of a large area on the western side of the city.
A rebel fighter who fled the Zawiya at the end of March said "there are clashes inside Zawiya itself".
The rebel, who identified himself only as Kamal, said "the fighters are back in the city."
Zawiya was the closest city to the capital Tripoli to fall into rebel hands.
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