BENGHAZI, Libya (Reuters) - Islamic State militants attacked checkpoints near the Libyan oil port of Es Sider for a second day on Tuesday and an oil storage tank in the port was set on fire by a long-range rocket, a spokesman for the security guards said.

Ali Hassi said militants had attacked checkpoints 30-40 km (19-25 miles) from the port, and that two guards were killed and 16 wounded in the fighting. Seven guards were killed and 25 were wounded in Monday’s clashes, he said.

The National Oil Corporation (NOC) said the oil tank fire started just as firefighters were close to bringing under control another blaze at an oil tank that was hit during fighting in the nearby port of Ras Lanuf on Monday.

Both fires were still burning on Tuesday afternoon.

Es Sider and Ras Lanuf, Libya’s biggest oil ports, have been closed since December 2014. They are located between the city of Sirte, which is controlled by Islamic State, and the eastern city of Benghazi.

Libya descended into chaos after the fall of Muammar Gaddafi in 2011 and rival governments and militias have been competing for the country’s oil wealth ever since.

The U.N. is trying to win support for a deal to form a national unity government, but many members of Libya’s rival parliaments have not signed up.

The country’s crude oil production has dropped to less than a quarter of a 2011 high of 1.6 million barrels per day.

Islamic State militants have taken advantage of a security vacuum to tighten their grip on Sirte, and have been threatening to advance east along the coast. They have not managed to take control of any oil installations yet, as they have in Syria.

On Monday, Islamic State suicide car bombers struck near Es Sider and there were clashes between its fighters and security guards. The tank that was hit in Ras Lanuf, 20 km (13 miles) from Es Sider, was holding about 400,000 barrels of oil.