The militia buildup "is something that definitely concerns us and something that we are watching very carefully," said Col. David Gray, commander of the 101st Airborne's 1st Brigade Combat Team, based in Kirkuk. "So far they haven't been that violent, but does it add to the tension, putting them into this maelstrom? Absolutely."
Observers have said should Iraq collapse into civil war, Kirkuk would be the likely flashpoint because of its lucrative oil supplies, the Washington Post reported.
However, Kurdish leaders now speak openly of using force if necessary to gain control of the city, which they consider the historical capital of a vast Kurdish nation also extending into Iran and Turkey.
Shiite residents of Kirkuk make up less than 5 percent of the local population, the report said.
2014: The Year in Music [PHOTOS]