Defense Secretary Ash Carter on Tuesday defended President Barack Obama's strategy to defeat Islamic State militants amid blistering criticism from Senate Armed Services Committee Chairman John McCain.

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 U.S. 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 Arizona 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."

McCain said Islamic State continues to gain territory in Iraq and Syria, while expanding its influence across the Middle East, Africa and Central Asia. He said there is no responsible ground force in either Iraq or Syria that can seize territory from IS, and slim prospects as well from the U.S.-led coalition's current training efforts of local ground forces.

Iraq is going through its worst crisis since the 2011 withdrawal of U.S. 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.

"While our coalition may own the skies, as the president said yesterday, our air campaign against IS continues to be limited significantly by overly restrictive rules of engagement and a lack of ground intelligence, which only gets worse as IS moves into urban areas to avoid coalition bombing," McCain said.

"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.

"The strategy is the right one, but its execution can and will be strengthened, especially on the ground," Carter said.

"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.

He acknowledged publicly for the first time that the administration's program for training moderate Syrian rebel fighters has slowed to the point where it has only about 60 fighters involved. The training is being undertaken at bases in Jordan and Turkey. The Associated Press reported last month that the number had dropped below 100, with dozens of recruits leaving the program.

"The number is much smaller than we hoped for at this point," Carter conceded Tuesday. He said, nonetheless, that it remains an essential element of the U.S. strategy. And Carter also the training has given the U.S. a window into the opposition groups inside Syria. That, he added, has yielded valuable intelligence and increased the Pentagon's ability to attract moderate rebel recruits.

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Associated Press Writer Robert Burns contributed to this report.