The plan is for the as-yet-undefined group of moderate Syrian rebels, supported by US and Turkish airstrikes, to seize a 68-mile stretch of the border and then keep out IS.
"We have to sit down with the Turks and figure it out," one official familiar with the situation was quoted as saying by antiwar.com.
The official added that the United States had a much easier time identifying rebel groups "we absolutely will not work with," but leaders seem to be struggling to figure out who that leaves.
One possible choice is the United States-trained New Syrian Forces, which was created to fight IS. However, American forces only managed to train 54 people, so they would likely struggle to maintain control of such a large area, if they could ever gain control in the first place.
Kurdish forces are more numerous in the region and are being backed by the Unites States, but Turkey has been fighting the Kurds as much as they have the IS militants.
From Turkey's perspective, al-Qaeda seems to be the least-objectionable rebel faction, but selling that idea to the United States may be difficult as well, antiwar.com reported.
On Tuesday, three senior Obama administration officials made it clear that the operation is aimed strictly at clearing out IS militants, and there are no US plans for a so-called “safe zone” designed to protect civilians.
"We're not out there staking out zones and doing some things that I know have been discussed in years past – no-fly zones, safe zones. What we're trying to do is clear ISIL," one official was quoted as saying by Bloomberg.
"I think it's important not to confuse that with staking out these zones that you can identify with road signs and on big maps, and that's just not what's happening."