The jihadist group Islamic State in Iraq and Syria (Isis) has shelled key oil facilities in the Libyan ports of El-Sider and Ras Lanuf, setting five oil storage tanks on fire, according to press reports.

The country’s Petroleum Facilities Guards are still in control of the faciltiies, a group spokesperson Ali al-Hassi told media on 6 January.

Firefighters are attempting to put out the blaze, which is affecting one tank in Ras Lanuf and four in El-Sider. Each storage tank is reported to contain more than 400,000 barrels of oil.

The two ports, which have a combined capacity of 550,000 barrels a day (b/d), closed in December 2014 due to fighting between rival government militias. Six storage tanks were set on fire on 25 December 2014, and burned for about 10 days.

Thirty Isis fighters and nine soldiers from the Petroleum Facilities Guards were killed in clashes on 4 and 5 January 2015, according to Al-Hassi. The guards received air support from the General National Council (GNC), the government based in Tripoli.

Isis attacked several oil fields including Mabruk, Bahi, Dahra and Ghani in 2015, causing them to be shut down and forcing state-owned National Oil Corporation (NOC) to declare force majeure on several of its assets.

The jihadist group has taken advantage of the civil war in Libya to take control of the central city of Sirte and Bin Jawad, 30 kilometres west of El-Sider. It is using these as staging points to attack the ports.

The attempt by Isis to take control of these oil facilities will renew the urgency of negotiations between the GNC and the internationally recognised government in the eastern coastal town of Baida to form a unity government.

A UN-brokered peace deal signed by the rival governments in December has not been implemented.