The force will be essentially a restructured version of the Syrian Democratic Forces, or S.D.F., its spokesman, Mostafa Bali, said Tuesday. He said the fighters would be “professionally well trained as border guards,” and would deploy along parts of Syria’s borders with Turkey and Iraq to prevent an Islamic State resurgence, which he called “a moral duty.”

It would also, he said, hold the line between S.D.F.-held territory and areas held by Syrian government forces, roughly along the Euphrates River.

The territory includes large swaths of land captured from the Islamic State by the S.D.F., the Kurdish and Arab militia that has been the United States’ main partner in Syria against the militant group. But there has never been agreement on what would happen to that territory once the Islamic State was vanquished.

The Kurdish party that dominates the S.D.F. has always said the territory would remain as a self-governing part of a federalized Syria. American officials have said the United States would continue to support its allies in Syria, but they have been vague about how and for how long.

The new force suggested a possible answer to that question.

But the Syrian government and its allies Russia and Iran object to any division of the country. So do most Syrian opposition groups. The government led by President Bashar al-Assad wants to re-establish control over all of Syria and objects to American tinkering outside of a negotiated peace deal.