Oil markets jumped on Monday as Iraqi forces entered the oil city of Kirkuk, taking territory from Kurdish fighters and briefly cutting some output from OPEC’s second-largest producer. Iraq launched the operation in the multi-ethnic region on Sunday as the crisis between Baghdad and the Kurdish Regional Government (KRG) escalated. Tensions have been building since the KRG voted for independence in a Sept. 25 referendum. On Monday, Iraq’s Kurdistan briefly shut down some 350,000 barrels per day (bpd) of production from major fields Bai Hassan and Avana due to security concerns, according to Reuters.

Brent crude futures LCOc1 were at $58.07 per barrel at 1338 GMT, up 90 cents from the previous close. U.S. WTI crude was at $52.07 per barrel, up 62 cents. “The escalation in Northern Iraq is the main driver,” Commerzbank analyst Carsten Fritsch told the Reuters Global Oil Forum. “Oil supply from this region is at risk.” The government said its troops had taken control of Iraq’s North Oil Company, and the fields quickly resumed production. The KRG government said oil continued to flow through the export pipeline, and that it would take no steps to stop it.

Still, the action unsettled the market. There is some 600,000 bpd of oil produced in the region, and Turkey has threatened to shut a KRG-operated pipeline that goes to the Turkish port of Ceyhan at Baghdad’s request.