BENGHAZI, Libya (Reuters) - Libya's eastern Es Sider oil export port, the country's biggest, remains closed due to poor security, oil officials said on Wednesday. State oil firm NOC closed the terminal and the nearby Ras Lanuf port when fighting erupted between forces allied to the country's two rival governments in December. A month ago, a state oil security force based at Es Sider and Ras Lanuf called on NOC to reopen the two ports, after a force loyal to a non-recognised rival government in Tripoli pulled out from the area following months of fighting. But a senior port official told Reuters the security situation all the way from Es Sider to Sirte, a major central city where Islamic State militants have taken over government buildings, was still too poor to receive tankers. "There are security risks from Es Sider until Sirte," he said, asking not to be named. A spokesman for NOC said force majeure remained in place for the two ports, which have a combined capacity of 600,000 barrels a day. NOC had declared the contractual waiver in December. The region south of the ports is home to several oilfields that were attacked in recent weeks by fighters loyal to Islamic State. Libya's western El Feel oilfield is still closed due to a strike by security guards, a spokesman for NOC said. El Feel, which analysts say produced about 100,000 barrels per day (bpd), is operated by a joint venture owned by NOC and Italy's Eni. Libya this year had managed to restart El Feel after it had to shut late last year when a group in the Zintan region, which opposes a self-declared government in Tripoli, closed a pipeline. Militants are exploiting a security vacuum, just as they did in Syria and Iraq, created by a struggle between two governments vying for control four years after the ousting of leader Muammar Gaddafi. The North African country's official government has been confined to the east since a rival group seized Tripoli in August, setting up its own administration. Libya's oil output is between 500,000 bpd and 600,000 bpd, analysts say. NOC had to declare force majeure this month at 11 oilfields in the Sirte basin south of Es Sider after Islamic State attacked at least three facilities and kidnapped up to 10 foreign oil workers.