After numerous clashes with government forces, the rebels were able to reach the outskirts of Brega, which has already changed hands many times since fighting began in March, Col. Hamid Hassy said.
Rebel forces maintained their position for four days around the city of Ajdabiya, allowing NATO airstrikes to weaken government forces. On Friday, the fighters pushed in and reached Brega's university campus, just outside the town's oil port.
Hassy said that if rebels retake Brega, they will bring engineers to repair any damage to the refinery and oil facilities there.
Government troops, meanwhile, continued their powerful assault with tanks and rockets on Misrata, the last major rebel city in western Libya.
Rebels in Misrata and the New York-based group Human Rights Watch have alleged that Moammar Gadhafi's forces have been using cluster bombs, which pose particular risk to civilians because they scatter small bomblets over a wide area. Most of the world's nations have banned the use of the munitions.
However, Libyan government officials have denied the use of cluster bombs.
U.S. Secretary of State Hillary Rodham Clinton said she was unaware of the reports about the use of cluster bombs.
"I have to say I am not surprised at anything that Col. Gadhafi and his forces do, but that is worrying information and it is one of the reasons why the fight in Misrata is so difficult," Clinton said. "It is at close quarters, it is in amongst urban areas and it poses a lot of challenges to both NATO and to the opposition."
The NATO-led air campaign has rebels from being outright defeated on the battlefield by the better trained and equipped government forces, but it still has not been enough to completely turn the tide.
So far, the airstrikes have not been enough to sufficiently degrade government forces to allow the rebels to reach Gadhafi's heavily defended hometown of Sirte, the gateway to the regime-controlled western half of the country.
In contrast to their pell-mell charges and retreats in the past six weeks, the rebels appear to be trying a more gradual advance that might actually result in them holding territory.
The Associated Press contributed to this report.