Meanwhile, U.S. aircraft conducted three more airstrikes against the militants, primarily ISIS-driven vehicles, near the Mosul Dam, the Pentagon said.

Since August 8, the U.S. military has carried out 93 airstrikes, 60 of them in support of Iraqi forces near the Mosul Dam, according to the Defense Department.

The dam is the largest in the country, and Iraqi and U.S. officials fear that a breach in the dam would threaten the lives of millions of Iraqis who live downstream in Mosul and Baghdad.

The U.S. airstrikes have continued despite ISIS' threat to kill another American hostage.

Earlier this week, ISIS posted to the Internet a video showing the execution of American journalist James Foley. The group threatened the life of another man, believed to be American journalist Steven Sotloff, if the United States didn't end airstrikes in Iraq.

Obama vowed justice in a televised address on Wednesday, saying the country would "continue to do what we must do to protect our people."