Production has been halted at Iraq’s biggest oil refinery and foreign staff has been evacuated from the facility in Baiji amid attacks by the al-Qaida splinter group making advances in the country, although the Iraqi military maintained control of the refinery.

“Due to the recent attacks of militants by mortars, the refinery administration decided to evacuate foreign workers for their safety and also to completely shut down production units to avoid extensive damage that could result," a chief engineer at the Baiji oil refinery told Reuters on Tuesday. He said the facility was shut down overnight.

Sunni militants with the Islamic State of Iraq and Syria, or ISIS, surrounded the refinery last week as it also took control of Iraq’s second-largest city, Mosul.

The refinery in Baiji is the largest in Iraq and processes oil from the northern portion of the country. The chief engineer at Baiji’s refinery said there is enough gas oil, gasoline and kerosene to supply domestic demand for more than a month. 

The Baiji refinery can reportedly process 300,000 barrels a day and supplies oil products to most of Iraq's provinces and Baghdad, according to Reuters.