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Libya's National Oil Corporation (NOC) said on Tuesday evening that all the fires sparked by drone attacks on the country's second-largest oil refinery had been contained.

On Monday night, a large gasoline reservoir west of the capital caught fire and collapsed after it was "struck by a drone, while another drone fell near a second tank without causing casualties", Hamdi al-Bishti, head of the Brega Oil Company that runs the refinery, told AFP.

The NOC said another drone had targeted the complex but did not cause any damage.

It remains unclear who is behind the attacks.

In a statement, the Tripoli-based government condemned the strikes as "an escalation that will not go unpunished", vowing to "hunt down those responsible".

"This is a direct threat to the lives of citizens and workers, as well as to vital infrastructure that forms an essential part of the country's security, economy and resources," Prime Minister Abdulhamid Dbeibah said.

Firefighting efforts and the cooling of adjacent tanks in the Zawiya oil refinery to prevent the fire from spreading had been underway since the night of the strike.

Bishti said it was difficult to fully contain the fire given the tank was filled with gasoline.

The NOC said the tank contained about 4.5 million litres of gasoline and declared a state of emergency Monday night.

The latest attacks came days after similar drone strikes targeted a naphtha reservoir and a water desalination plant over the weekend.

"It's a heinous act," said Bishti. "We came close to tragedy."

Libya has the most abundant oil reserves in Africa but has been beset by violence and instability since the overthrow of Muammar Gaddafi in 2011.

Two governments are currently vying for power: the UN-recognised administration in Tripoli led by Dbeibah and a rival in the east backed by military commander Khalifa Haftar.

The reservoir is part of the second-largest oil refinery in Libya, with a refining capacity of 120,000 barrels per day.

Brega said the site held significant amounts of diesel, kerosene, gasoline and liquefied petroleum gas (LPG) -- petroleum derivatives intended to fuel power stations in western Libya.