WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Friday that tensions between Kurdish and Iraqi forces in and around Kirkuk had the full attention of the United States, which was working to ensure it does not escalate.

Kurdish Peshmerga fighters ride in a vehicule in the Southwest of Kirkuk, Iraq October 13, 2017. REUTERS/Ako Rasheed

Kurdish authorities said they have sent thousands more troops to Kirkuk to confront “threats” of Iraqi military attack, but also pulled back defense lines around the disputed oil-producing area slightly to ease tensions.

“We have got to work on this, the secretary of state has the lead, but my forces are integrated among these forces and they are working too, to make certain we keep any potential for conflict off the table,” Mattis told reporters.

The Baghdad central government has taken a series of steps to isolate the autonomous Kurdish region since its overwhelming vote for independence in a Sept. 25 referendum, including banning international flights from going there.

Mattis said while he was aware of troop movements, he had not heard of any fighting and called on both sides to focus on fighting Islamic State militants.

“We can’t turn on each other right now. We don’t want this to go to a shooting situation,” Mattis added.

Kirkuk, a city of more than 1 million people, lies just outside Kurdish territory, but Peshmerga forces deployed there in 2014 when Iraqi security forces collapsed in the face of an Islamic State onslaught. The Peshmerga deployment prevented Kirkuk’s oil fields from falling into jihadist hands.

As the territory controlled by Islamic State has shrunk, ethnic and sectarian fractures that have plagued Iraq for more than a decade have once again started to resurface.

The group’s last territory in Iraq is now a stretch skirting the western border with Syria following the fall of the town of Hawija and surrounding areas on Oct. 5 in an offensive by U.S.-backed Iraqi forces.

Mattis said the differences would have to be worked out politically and not on the battlefield.

“These are issues that are longstanding in some cases. ... We’re going to have to recalibrate and move these back to a way (where) we solve them politically and work them out with compromised solutions,” he said.