Pumping stopped at Kirkuk's two main oil fields Monday as Iraqi forces advanced in the disputed region following tensions over a Kurdish independence vote, an oil ministry official said.

"The Kurdish technicians stopped pumping operations at the wells of Bay Hassan and Havana and left the wells before the arrival of Iraqi forces," said the official.

Oil prices spiked on world markets after Iraqi forces launched the operation overnight to retake military facilities and oil fields the Kurds had seized in recent years, mainly during the fight against jihadists.