Iraqi army units advanced overnight Sunday toward Kirkuk, a major oil city that’s dominated by the country’s Kurdish people and which recently declared independence.

The Iraqi national army has taken control of “vast areas” without opposition from Kurdish peshmerga fighters, Reuters reported, citing state television. There were no confirmed reports of fighting.

Oil prices reacted strongly to the news, with Brent crude rising more than 1 percent to $57.77 a barrel during Asian trade on Monday. U.S. oil futures meanwhile was 0.8 percent higher, hovering near the $52 level.

“Just as the battle against ISIS seems to be finally ending, there is a new theater of battle emerging in Northern Iraq,” John Kilduff, partner at energy-focused investment manager Again Capital, told CNBC.

The maneuvers come after Iraq’s semi-autonomous Kurdistan region held an independence referendum last month.

The Kurds are a separate ethnic group from the Arabs and are primarily Sunni Muslims. The Iraqi army is dominated by Arabs who are Shiite Muslims.

In a Friday research note, risk consulting firm Eurasia Group warned that Iraqi Prime Minister Haidar al Abadi “is increasingly committed to re-establishing central government control over the territories contested by Baghdad” and the Kurds.

Iraqi forces are looking to take over the Kurdish-controlled K-1 airbase, which used to be a former a Iraqi air force base, a senior official told Reuters on Sunday.

A map of Kirkuk, showing the close proximity of the K-1 airbase, marked at upper left. Courtesy Google Maps.

The Kurds have pressed for their own nation state for more than a century, but that movement gained momentum after the U.S. invasion of Iraq in 2003 and the rise of the so-called Islamic State, or ISIS, that followed.

Kurdish fighters were among the most effective troops for the Iraqi government during the Iraq War, and they have easily been the most successful force on the ground against ISIS, which swept the regular Iraqi army from the field in 2014.

“In the summer of 2014, Kurdish forces exploited the collapse of the Iraqi army in northern parts of the country to move into areas claimed both by the region and by federal authorities, especially oil-rich Kirkuk. The central government remains unlikely to accept this presence,” Eurasia Group said.

Kurdistan includes parts of Iraq, Iran, Turkey and Syria. Kurdish independence is opposed by every major player in the region — even including the United States, which has fought closely alongside the Kurds since 2003.

“The Kurds have no friends — Turkey, Iraq, Iran, Russia, and the U.S., among others, have decried their independence push,” Kilduff said.

Iraq is the second-biggest oil producer in OPEC. Kurdish-controlled areas of Iraq are among the most productive in the country and contain much of its energy infrastructure.

“Oil prices could spike a lot higher on this development because this time is different, after years of war in the region. The battle, finally, is for the oil, and no other reason. In other words, here we go,” Kilduff said.

Kilduff added that oil infrastructure, which was largely spared in previous fighting, “will likely be the main target this time around.”

The Pentagon has urged both sides to “avoid additional escalatory actions” and warned that it opposed any destabilizing actions that detracted the fight against Islamic State militants, Reuters reported on Monday.

—CNBC’s Patti Domm contributed to this report