The manager of the installation said they took control at dawn without using force, but rebel forces later said they had regained control of the town.
Pro-Gaddafi jets also bombed an arms dump in the nearby city of Ajdabiya.
The city’s defenders are in a high state of excitement and expecting an attack, a BBC correspondent says.
In two weeks of unrest Col Gaddafi has lost control of large parts of Libya.
The violence has led to a major humanitarian crisis on the Tunisian border, with tens of thousands of foreigners, most of them Egyptian, stranded and unable to get home.
Some 75,000 people have fled to Tunisia since unrest began and 40,000 more are waiting to cross, the UN says.
The United Nations says a mass evacuation is needed and thousands of lives are at stake.
Here in Ajdabiya, an imminent attack doesn’t seem very likely. Still, in the last few minutes there was the second bombing raid of the morning by a pro-Gaddafi pilot, on the enormous arms dump on the edge of town here.
That makes the fifth attack on the arms dump in 12 days, though no real damage has so far been caused to it.
Rebels determined
Government forces took the oil facility at Brega at dawn on Wednesday without using force.
“It’s not an attack. We are OK. The government troops came in to secure the whole area. Our concern is to maintain the facility,” Ahmed Jerksi, the manager of the oil installation in Brega, told the Associated Press.
But rebels in Benghazi, the main city in eastern Libya, said they had retaken the town.
“They tried to take Brega this morning, but they failed,” spokesman Mustafa Gheriani told Reuters news agency. It is back in the hands of the revolutionaries. He is trying to create all kinds of psychological warfare to keep these cities on edge.”
However, government spokesman Mussa Ibrahim told the agency he doubted there had been a government attack on Brega.
The BBC’s John Simpson in Ajdabiya says rebel forces in Ajdabiya have been expecting an attack but with fighting continuing in Brega that may not be imminent.
The rebels are determined to put up a fight but it remains to be seen whether this translates into an organised defence of the city, our correspondent says.
Protesters fear air attacks on the towns they have won and are calling for the creation of a no-fly zone over Libya.
Watch: UN Secretary General Ban Ki-moon said that the world had spoken with “one voice”
Meanwhile, UN Secretary General Ban Ki-moon said reports suggested more than 1,000 had died in the unrest so far.
The UN has passed a resolution suspending Libya from its Human Rights Council and accusing it of committing gross and systematic violations of human rights.
Mr Ban said: “These UN actions send a strong and important message – a message of great consequence within the region and beyond – that there is no impunity, that those who commit crimes against humanity will be punished, that fundamental principles of justice and accountability shall prevail.”