The attack on Brega marked the regime's first counteroffensive on the eastern half of the country, which fell quickly under opposition control after the revolt began on Feb. 15.
At least 14 people have been killed.
Brega is the second-largest petroleum and natural gas facility in OPEC-member Libya and has been held by the opposition since last week.
Opposition leaders are pleading for a "no-fly zone" and for foreign powers to launch airstrikes to help them oust Libyan leader Moammar Gadhafi.
But the Pentagon tried to play down the idea of using military force in Libya. Defense Secretary Robert Gates said a "no-fly zone" would first require attacking Gadhafi's government.
In the meantime, the International Criminal Court will investigate Gadhafi and his inner circle for possible crimes against humanity.
Gadhafi's regime has unleashed the bloodiest crackdown of any Arab nation to the wave of anti-government protests in the region. Hundreds are known to have been killed, and some estimates top 1,000.
The Associated Press contributed to this story.