Armed militants from the hard line Islamic State in Iraq and the Levant (ISIL) overran the Ajeel oil site, 30 km (20 miles) east of Tikrit, according to Reuters.
The site contains at least three small oilfields that produce 28,000 barrels per day, an engineer working at the field said.
The engineer said local tribes had taken responsibility for protecting the fields after police withdrew, but that they also left after the nearby town of al-Alam was seized by militants.
Ajeel is connected to two pipelines, one running to Turkey’s Ceyhan port and the other to the Baiji oil refinery, which remained a frontline early on Wednesday.
State TV showed troop reinforcements being flown into the compound by helicopter to fend off the assault on Baiji, a strategic industrial complex 200 km north of Baghdad.
Local tribal leaders said they were negotiating with both the Shi’ite-led government and Sunni fighters to allow the tribes to run the plant if Iraqi forces withdraw. One government official said Baghdad wanted the tribes to break with ISIL and other Sunni armed factions, and help defend the compound.
The plant has been fought over since last Wednesday, with sudden reversals for both sides and no clear winner so far.
