The Benghazi Defence Brigades (BDB) says it has captured five cities and two major oil-producing areas in the east of Libya after opposing forces carried out air raids around major oil ports overnight, seeking to regain control of the area.

At least nine men were killed in the fighting on Friday as the eastern-based Libyan National Army (LNA) and allied forces retreated from the oil ports of Es Sider and Ras Lanuf, two of Libya's largest export terminals, following the attacks by the BDB.

The forces of eastern Libya's military strongman Khalifa Haftar conceded the loss on Saturday.

"The attackers were armed with modern tanks," spokesman for Haftar's forces, Colonel Ahmad al-Mismari, said.

"We lost two men. But the battle is ongoing. The situation in the Oil Crescent remains under control."

The assault raised the prospect of a new escalation of violence around the ports, and put at risk a sharp boost to Libya's oil production achieved after the LNA took over four ports in September, ending a blockade at three of them.

Though Es Sider and Ras Lanuf have been reopened for exports, they were badly damaged in past fighting and are operating well below capacity.

It was not clear to what extent the BDB had gained control over the ports, or whether operations at the terminals had been affected.

"Haftar claims to control most of the eastern part of Libya around Benghazi," Oliver Miles, former British ambassador to Libya, told Al Jazeera.

"But, control is a loose word. He's been fighting for a long time to deal with them and it has taken a long time time to clear them up. He's repeatedly said they've dealt with the problem but they keep reuniting."

In response to the BDB advance, air raids were carried out in Ras Lanuf, Es Sidra, Ben Jawad and Harawa, Mismari added.

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A resident in Ras Lanuf said they heard fighter jets over the town at dawn on Saturday, followed by explosions.

Residents posted pictures of fires and damage to buildings apparently caused by the strikes.

Libya's oil production has recently been fluctuating around 700,000 barrels per day (bpd), more than double its output last year but still well under the 1.6 million bpd the OPEC member was producing before a 2011 uprising.

The Benghazi Defence Brigades are composed partly of fighters who were ousted from Benghazi by the LNA.

That battle is linked to a wider conflict between political and armed factions based in eastern and western Libya.