Iraqi Kurds prepare for independence referendum that could impact fight against ISIS

— -- Over 5 million Iraqi Kurds are preparing to hold an independence referendum that could have significant consequences for the fight against ISIS. The referendum, scheduled for Sept. 25, will determine if Iraq's Kurdistan region in the north will break away from the Iraqi government.

The U.S. fears the decision could disrupt the war due to the important role Kurdish Peshmerga, fighting alongside Iraqi Security Forces, have played in recapturing Iraqi cities from ISIS. The fighting forces, backed by the U.S.-led coalition, have steadily regained territory -- most recently crushing ISIS fighters in Tal Afar in an 11-day battle.

Several top U.S. officials have voiced their concerns, including asking the head of the Kurdish Regional Government to postpone the vote to maintain focus on the ISIS fight.

On Aug. 22, during a trip to Iraq, Secretary of Defense James Mattis encouraged dialogue between Kurdish President Masoud Barzani and Iraqi Prime Minister Haider al-Abadi and asked all sides to "keep the focus on maintaining the momentum against ISIS" -- a sentiment shared by the State Department and U.S. Central Command (CENTCOM), which oversees the coalition.

"ISIS is the main fight that Iraqis have been fighting for years now," State Department spokesperson Heather Nauert said on Aug. 15, adding, "We see that as the sole focus where we need to stay -- where we need to keep the eye on the ball," she said.

"We'd prefer everyone stayed fully focused on the fight against ISIS," Col. John Thomas, CENTCOM spokesman, told ABC News on Wednesday when asked about the referendum.

Brett McGurk, the U.S. special envoy for the anti-ISIS coalition, has had some of the strongest language against the referendum, calling its timing "potentially catastrophic to the counter-ISIS campaign."

"I would just mention that is not just the United States that does not think this referendum should be held," he said on Aug. 21 during a joint press availability with Mattis in Jordan. "It’s really every member of our coalition who believes now is not the time to hold this referendum. So we’ve made our position on that really clear."

Turkey and Iran have also voiced their opposition, with Turkey's Ministry of Foreign Affairs calling the vote a "grave mistake."

Despite Iraq's government rejecting the referendum in a non-binding resolution, Barzani defended it in Kirkuk on Tuesday, saying the vote is "entirely legal."

Kirkuk province, which is home to Arabs, Kurds, Turkmen and Christians, is controlled by Kurdish forces but claimed by the Iraqi government. Barzani promised that the area, which will also participate in September's referendum, will retain its identity.

The push by Iraq's Kurds for independence dates back over a decade.

In 2005, Kurdish leaders wanted to add a clause to Iraq's constitution that would allow for a referendum. The vote that is scheduled for this September was originally supposed to take place in 2014 but was repeatedly delayed.

In that time, the Kurds have taken advantage of the opportunity to push out ISIS, gaining additional territory and bolstering their case for an independent state.

The Kurds are one of the largest ethnic groups in the Middle East and have their own language and cultural traditions. There are large populations of Kurds in Iraq, Syria, Turkey, and Iran.

ABC News' Sarah Kolinovsky contributed to this report.