Iraq's Kurds are sticking to a plan to hold an independence referendum on Sept. 25, despite a U.S. request to postpone it, a high-ranking Kurdish official told Reuters on Saturday.

The United States and other Western nations are worried that the vote could ignite a fresh conflict with Baghdad and turn into another regional flashpoint. Turkey, Iran and Syria, which together with Iraq have sizeable Kurdish populations, all oppose an independent Kurdistan.

"The date is standing, Sept. 25, no change," said Hoshyar Zebari, a close adviser to Kurdistan Regional Government President Massoud Barzani, after U.S. Secretary of State Rex Tillerson asked Barzani to postpone the referendum.

Tillerson made the request in a phone call with Barzani on Thursday, Zebari said.

"On the issue of the postponement of the referendum, the President (Barzani) stated that the people of the Kurdistan Region would expect guarantees and alternatives for their future," said a statement issued on Friday by the Kurdistan Regional Government (KRG) presidency, after Tillerson's call.

The U.S. State Department said in June it was concerned that the referendum will distract from "more urgent priorities" such as the defeat of Islamic State militants.

Islamic State's self-proclaimed "caliphate" effectively collapsed last month, when U.S.-backed Iraqi forces completed the takeover of the militants' capital in Iraq, Mosul, after a nine-month campaign in which Kurdish Peshmerga fighters took part.

The hardline Sunni militants remain however in control of territory in western Iraq and eastern Syria. The United States has pledged to maintain its backing to allied forces in both countries until the militants' total defeat.

The Kurds have been seeking an independent state since at least the end of World War One, when colonial powers divided up the Middle East, but their territory ended up split between modern-day Turkey, Iran, Iraq and Syria.

Barzani, whose father led struggles against Baghdad in the 1960s and 1970s, told Reuters in July the Kurds would take responsibility for the expected 'yes' outcome of the referendum, and pursue its implementation through dialogue with Baghdad and regional powers to avoid conflict."We have to rectify the history of mistreatment of our people and those who are saying that independence is not good, our question to them is, 'if it's not good for us, why is it good for you?'," he said in an interview in the KRG capital, Erbil.

Iraq has been led by Shi'ites since the overthrow of Saddam Hussein, a Sunni, by the U.S.-led invasion of 2003.

The country's majority Shi'ite community mainly lives in the south while the Kurds and Sunni Arabs inhabit two corners of the north. The center around Baghdad is mixed.

Kurdish officials have said disputed areas, including the oil-rich Kirkuk region, will be covered by the referendum, to determine whether they would want to remain or not in Kurdistan.

The Kurdish Peshmerga in 2014 prevented Islamic State from capturing Kirkuk, in northern Iraq, after the Iraqi army fled in the face of the militants. They are effectively running the region, also claimed by Turkmen and Arabs.

Hardline Iran-backed Iraqi Shi'ite militias have threatened to expel the Kurds by force from this region and three other disputed areas - Sinjar, Makhmour and Khanaqin.