Rebels had advanced to the outskirts of Brega on Saturday, but government troops are pushing them back.
Gadhafi's troops have been shelling Ajdabiya, the city that serves as a gateway to the rebel-held east.
Officials said the fighting has left seven rebels dead, 27 wounded and four missing.
In March, Gadhafi's troops encircled Ajdabiya with tanks, armored personnel carriers and heavy artillery before NATO airstrikes decimated the forces besieging the city and allowed the rebels to reclaim the town and push west.
The NATO-led air campaign has kept rebels from being defeated on the battlefield by the better equipped government forces, but it still has not been enough to completely turn the tide. The rebels have been unable to reach Gadhafi's heavily defended hometown of Sirte, the gateway to the regime-controlled western half of the country.
Rebel advances west of have ultimately floundered as rebels overextended their supply lines and were routed by the heavier firepower and more sophisticated tactics of the government forces.
But while Gadhafi's troops have been able to halt rebel advances and push back east, they have been unable to move in on Benghazi, largely because of the threat of NATO airstrikes on Gadhafi's exposed forces.
The Associated Press contributed to this report.