Moving forward with the referendum on Sept. 25 would be a "risky" move for Iraq's autonomous Kurdish region because there was no international support for it at this moment, McGurk said in a news conference in Irbil, Iraq.
He noted that Belgium, Britain, France and Iraq had cooperatively developed an "alternative plan" to the referendum.
McGurk did not provide details of the "alternative plan" but he said he presented it to KRG leaders.
Since the KRG announced its plan to hold the referendum, it has gained little support from the international community and has instead been met with harsh criticism.
The U.S. and other Western nations fear that the Sept. 25 vote in Iraqi Kurdistan could ignite a new conflict with Baghdad and possibly neighboring countries, diverting attention from the war against Daesh terrorists in Iraq and Syria.