"When you see somebody killed in such a horrific way, that represents a terrorist attack — that represents a terrorist attack against our country and against an American citizen," Deputy National Security Advisor Ben Rhodes said.
General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, said officials were worried about the possibility that European or U.S. nationals, radicalized after fighting in Iraq or Syria, would return to their home countries.
Dempsey suggested Islamic State would remain a danger until it could no longer count on safe havens in areas of Syria under militant control.
"This is an organization that has an apocalyptic, end-of- days strategic vision and which will eventually have to be defeated," Dempsey said.
"To your question, can they be defeated without addressing that part of their organization which resides in Syria? The answer is no. That will have to be addressed on both sides of what is essentially at this point a non-existent border."