Islamic State (IS) militants have set fire to oil wells northeast of the city of Tikrit, a witness said, to obstruct an assault by Shia militia fighters and Iraqi soldiers trying to drive them from the Sunni Muslim city and surrounding towns.
The witness and a military source said IS fighters ignited the fire at the Ajil oil field to shield themselves from attack by Iraqi military helicopters.
The offensive is the biggest Iraqi forces have yet mounted against IS, which has declared an Islamic caliphate on captured territory in Iraq and Syria and spread fear across the region by slaughtering Arab and Western hostages and killing or kidnapping members of religious minorities like Yazidis and Christians.
Black smoke could be seen rising from the oil field since Wednesday afternoon, said the witness, who accompanied Iraqi militia and soldiers as they advanced on Tikrit from the east.
Control of oil fields has played an important part in funding IS, even if it lacks the technical expertise to run them at full capacity.
Before it took over Ajil last June, the field produced 25,000 barrels per day of crude that were shipped to a Kirkuk refinery, as well as 150 million cubic feet of gas per day.
Interestingly, it is Tehran and not Washington that has been the key player in the current offensive, with Iranian Revolutionary Guard general Qassem Soleimani seen directing operations on the eastern flank, and Iranian-backed militia fighters leading much of the operation.