"Our men were tricked. Gaddafi's soldiers pretended to surrender, coming with a white flag, and then they fired on us," Mussa al-Mograbi said by telephone.
About 20 rebels were wounded and transferred to hospital in Ajdabiya, 160 kilometres (100 miles) south of Benghazi, the de facto capital of rebels who have been fighting to overthrow Gaddafi since mid-February, he said.
Rebels have spent months trying to seize the strategic oil hub of Brega, which would open the road to Sirte, Kadhafi''s hometown, and from there to the capital Tripoli.
Brega boasts an important oil refinery which, once operational, could supply the east of the country with much-needed fuel to produce electricity.
French and British attack helicopters have been backing rebels by targeting positions of pro-Gaddafi forces in and around Brega since June 3.
Rebels, who refuse to specify how many fighters have joined their ranks, estimate that some 5,000 loyalist troops are defending Brega.