At least one other American is being held hostage in Syria, White House Press Secretary Joshua Earnest said during his daily briefing.

Earnest was asked if 26-year-old Kayla Mueller, who died during Jordanian airstrikes against the self-styled Islamic State in Syria, was the last American believed to be in the hands of the Sunni militants.

"Sadly, I cannot say that," Earnest said. "There is at least one other hostage that is held in the region."

Earnest would not say if the hostage was being held by the Islamic State but he said that at least one person was being held in Syria.

The Islamic State has killed three other American hostages: American journalists Steven Sotloff and James Foley and aid-worker Peter Kassig. Mueller was being held by the militants in a building that was targeted by the American-led coalition.

Earnest said giving more details about the other American hostage is not in their interest.

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