WASHINGTON—The U.S. has returned about two dozen suspected Islamic State fighters detained in Syria to their home countries as it grapples with the fate of hundreds more, leaving the door open to sending some to the American prison at Guantanamo Bay, Cuba, U.S. defense officials said.

The Pentagon has been assisting the Kurdish and Arab forces it backed in Syria in holding about 600 Islamic State detainees in seven detention facilities there, U.S. defense officials said. The Defense Department has paid to upgrade the facilities...