Military strongman General Khalifa Haftar on Sunday launched an offensive on Libya's "oil crescent" along the northern coast.
His forces fought off guards to gain control of the oil terminals at Ras Lanuf, Al-Sidra and Zuwaytania, and they finally secured Brega unopposed on Tuesday.
"We have taken control of the Brega port completely and without any fighting," said Colonel Muftah al-Muqarief, who heads oil guards loyal to Haftar.
"The entire oil crescent region is now under our control," Muqarief told AFP.
Brega was taken largely thanks to "mediation involving residents and town elders", he said.
The loss of Brega is a fresh blow to the GNA.
Haftar, 73, who sees himself as Libya's saviour after battling jihadists out of most of second city Benghazi in the east, backs the parliament which has refused to grant the GNA a vote of confidence.
The Brega takeover comes hours after the United States and its major European allies -- which back the GNA -- condemned Haftar's offensive, echoing remarks by UN special envoy Martin Kobler.
"I call for the respect of UN Security Council Resolution 2259 which recognises the Government of National Accord... as the sole executive authority in Libya," Kobler said Monday.
Oil is Libya's main natural resource with reserves estimated at 48 billion barrels, the largest in Africa, but production has plummeted since 2011 as the country plunged into turmoil.
- 'Withdraw, without preconditions' -
On Sunday, Haftar's forces took Al-Sidra and Ras Lanuf ports before attacking Zuwaytina to the east.
The "oil crescent" lies between Benghazi and Kadhafi's hometown of Sirte, where pro-GNA forces have been battling the Islamic State group since May.
It was the first time Haftar's forces and fighters loyal to the GNA clashed directly since the unity government started working in the capital in March.
Kobler said oil installations must remain under the authority of the GNA's presidential council and stressed that Resolution 2259 "contains a clear prohibition on illicit oil exports".
Meanwhile, Britain, France, Germany, Italy, Spain and the United States condemned the attacks on the four oil ports and called on Haftar's forces to withdraw.
These powers are major players in the struggle to reunify and rebuild Libya, and insisted in a joint statement that the country's oil should be administered by the GNA.
"We call for all military forces that have moved into the oil crescent to withdraw immediately, without preconditions," they said.
The embattled GNA called on loyalist forces to "protect and defend" the ports against what it called a "flagrant aggression" of Libya's sovereignty.
The National Oil Commission, the country's main governmental oil institution, said it would start work on resuming exports from the ports in the oil crescent area in a statement on its website Tuesday night.
"Our technical teams already started assessing what needs to be done to lift force majeure and restart exports as soon as possible," NOC president Mustafa Sanalla said.
But the statement from the governmental institution did not explain how it could export oil from ports controlled by hostile forces.
The head of the rival government, Abdullah al-Thani, earlier said from the eastern city of Bayda that his administration "will work on the oil ports resuming work as soon as possible so as to guarantee all Libyans a decent life".
The head of the parliament based in the east, Aguila Saleh, said that Haftar's forces would withdraw and hand over the ports to the National Oil Corporation to resume oil exports.
But the NOC is split into two rival branches. The branch that issued the statement Tuesday is loyal to the GNA, but the other is aligned to the administration that Haftar supports.