Recent U.S. surveillance flights over northeastern Nigeria showed what appeared to be large groups of girls held together in remote locations, raising hopes among domestic and foreign officials that they are among the group that Boko Haram abducted from a boarding school in April, U.S. and Nigerian officials said.

The surveillance suggests that at least some of the 219 schoolgirls still held captive haven't been forced into marriage or sex slavery, as had been feared, but instead are being used as bargaining chips for the...