McCain, who lost to Obama in the 2008 presidential election and has been a harsh critic of the administration's foreign policy, said there is no compelling reason to think that anything the US is doing will be enough to achieve the president's goal of degrading and ultimately destroying IS.
"Our means and our current level of effort are not aligned with our ends," the Republican told Carter at a Capitol Hill hearing. "That suggests we are not winning, and when you are not winning in war, you are losing."
Iraq is going through its worst crisis since the 2011 withdrawal of US troops. The Islamic State group controls large swaths of the country's north and west after capturing Iraq's second-largest city of Mosul and the majority of the western Anbar province last year.
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"Pilots will tell you that they are only as good as the targets they receive," he said, and when "three-quarters of our air missions against IS still return to base without dropping weapons, that is indicative of a fundamental problem with our air campaign."
Carter backed the president's policy while acknowledging that more needs to be done to strengthen the mission to train local forces to combat the Islamic extremists.
"I've told Iraqi leaders that while the United States is open to supporting Iraq more than we already are, we must see a greater commitment from all parts of the Iraqi government," Carter said.