“We shut down most of the production units in the refinery as a precautionary measure. Now things are under control and we will re-start the shutdown units but we need 48 hours to restore full operation,” said the Baiji refinery official.

The pipeline brings natural gas from Kirkuk’s gas field to operate some of its units at the plant, which is about 180 km north of Baghdad.

The official said initial reports indicated the blast was caused by a rupture in a part of the pipeline that was buried underground and had rusted.

Abdulrahman Al-Jubouri, an engineer at the water treatment unit of the refinery, said the explosion might have been caused by a short circuit in electrical wires.

“The electricity cables are situated alongside the gas pipes and because the gas pipes are old, with many leaks, we think leaked gas, along with an electricity short circuit, may have led to the explosion. We are still working on extinguishing the fire and the real cause of the explosion is not clear,” he said.

The Baiji refinery was shut down completely for two days in February when militants attacked it, killing four workers and detonating bombs that touched off a raging fire.

A plot to bomb the refinery in March was foiled by Iraq’s security forces, who last Friday also defused make-shift bombs placed inside the Doura refinery in southern Baghdad. Iraq’s army and police are preparing to take full control of security ahead of a complete withdrawal of US troops by year-end.

While overall violence has dropped since the peak of sectarian fighting in 2006-7, bombings and attacks remain a daily occurrence.

Iraq signed a series of oil contracts with international firms that are expected to propel the OPEC nation into one of the world’s top oil producers.