BENGHAZI/TRIPOLI, Libya (Reuters) - Islamic State militants set fire on Thursday to oil storage tanks in a fresh assault on Ras Lanuf terminal in northern Libya and the group threatened further attacks as they exploit a prolonged power vacuum in the large north African nation.

A general view of the industrial zone at the oil port of Ras Lanuf in this March 11, 2014 file photo. REUTERS/Esam Omran Al-Fetori/Files

The chairman of the National Oil Corporation, Mustafa Sanalla, told reporters in Tripoli that Ras Lanuf - shut since December 2014 - would remain closed for a “long time” because of the damage inflicted on Thursday and in earlier attacks.

Libya remains dogged by violence and political turmoil nearly five years after the overthrow of veteran leader Muammar Gaddafi, with two rival governments and parliaments based in Tripoli and in the east as well as various armed factions vying for power and a share of the country’s oil wealth.

The Islamic State militants drove into the oil storage site early in the morning and clashed with security guards before retreating and firing from a distance to set four tanks on fire, NOC spokesman Mohamed al-Harari said.

A pipeline leading from the Amal oil field to the nearby Es Sider terminal, the biggest on Libya’s Mediterranean coast, was also targeted, said Mohamed al-Manfi, an energy official allied with Libya’s eastern-based government.

Ras Lanuf and Es Sider together have an export capacity of 600,000 barrels per day. They were processing about half of that before they were both closed in December 2014.

The NOC said the area was facing an “environmental catastrophe”, with huge columns of smoke billowing from the fires and damage to power lines supplying residential and industrial districts.

“Residents are trying to build a barrier to stop the oil and fire from reaching gas pipelines and water pipelines, and the main road,” the NOC’s Harari said.

Islamic State militants have managed to establish a foothold in the city of Sirte, which lies about 200 km (125 miles) along the coast to the west of Ras Lanuf and Es Sider.

In a video posted on Islamic State’s official Telegram channel, fighter Abu Abdelrahman al-Liby said: “Today Es Sider port and Ras Lanuf and tomorrow the port of Brega and after the ports of Tobruk, Es Serir, Jallo, and al-Kufra.”

OIL PRODUCTION DISRUPTED

Libya’s current oil production stands at 362,000 barrels per day, he told Reuters. That is less than a quarter of a 2011 high of 1.6 million barrels per day, though production has not changed significantly in recent weeks.

Two weeks ago clashes between Islamic State and the Petroleum Facilities Guards who control the area around Es Sider and Ras Lanuf left seven oil storage tanks damaged by fire and at least 18 guards dead.

At least 1.3 million barrels of oil were lost as a result of the clashes and up to 3 million barrels could be at risk because of the latest attack, said NOC spokesman Harari.

The NOC sent a tanker to remove oil from the terminals in an effort to prevent further damage, but guards prevented it from loading, citing security concerns.

On Thursday the NOC blamed the “intransigence” of the Petroleum Facilities Guards in blocking the shipment for the further damage it suffered from the latest attack.

The guards are led by a federalist who has supported Libya’s eastern government, but analysts say their loyalties are uncertain within the country’s complex pattern of allegiances.