VIENNA - Libya's oil output has been slashed to between 600,000 and 700,000 barrels per day (bpd) from more than one million following clashes at its Ras Lanuf and Es Sider oil terminals, a Libyan oil source said on Wednesday.

The National Oil Corporation (NOC) is looking at options to divert some oil exports from Ras Lanuf to Brega and Zueitina terminals, the source said.

Ras Lanuf and Es Sider have been closed since June 14, when armed factions opposed to Khalifa Haftar's Libyan National Army (LNA) attacked the two ports, forcing the NOC to close them and declare force majeure on exports.

The attack has caused a drop in oil production of 450,000 bpd, NOC Chairman Mustafa Sanalla said on Wednesday.

He added that NOC subsidiary Arabian Gulf Oil Company (AGOCO) had also lost an unspecified amount of production due to a technical problem. AGOCO exports crude oil from Hariga terminal in Libya's far east.

Two oil storage tanks at Ras Lanuf, tanks No. 2 and 12, were set on fire during the oil crescent fighting, shrinking storage capacity there by 400,000 barrels.

An NOC team visited the site to asses the damage and reported that one of the storage units was still burning at a low level, Sanalla said.

More than half the storage tanks at Ras Lanuf and Es Sider had been damaged in earlier fighting, reducing capacity at two of Libya's biggest export terminals.

Brega and Zueitina, which are located east of Ras Lanuf and Es Sider in Libya's eastern oil crescent, have remained operational and under the control of the LNA.

The LNA has said it is preparing a counter offensive to take back Ras Lanuf and Es Sider, launching a series of air strikes against their rivals in the area.

The NOC has blamed the attack on the terminals on militias led by Ibrahim Jathran, who blockaded oil crescent ports for several years before losing control of them in September 2016 to the LNA.

The LNA has also blamed the Benghazi Defense Brigades, a grouping of anti-Haftar fighters that has previously tried to take the oil crescent and advance on Benghazi, which has been fully controlled by Haftar since late last year.

(Writing by Aidan Lewis; editing by Catherine Evans and Jason Neely) ((Aidan.Lewis@tr.com; +216-29850352;))