ERBIL, Iraq — Defense Secretary Jim Mattis asked the president of Iraq’s Kurdish autonomous region on Tuesday to postpone a referendum next month on Kurdish independence.

Mr. Mattis’s meeting with President Massoud Barzani was part of a broader push by American officials to head off what they fear may be a new period of political instability as Iraq continues its battle against the Islamic State.

“Our point right now is to stay focused like a laser beam on the defeat of ISIS and to let nothing distract us,” Mr. Mattis told reporters, using an acronym for the Islamic State, shortly before flying to Erbil, the capital of the autonomous region.

Secretary of State Rex W. Tillerson made a similar appeal for the Kurds to postpone the referendum, which is scheduled for Sept. 25, in a telephone call earlier this month with Mr. Barzani. So did Gen. Joseph L. Votel, the head of the United States Central Command, who visited Iraq last week.