Former Defense Secretary Robert Gates on Sunday called President Obama's goal to destroy the Islamic State in Iraq and Syria (ISIS) “unrealistic” and warned that ground troops may be needed to “roll back ISIS.”

“I think that the air strikes have contributed to containing them, but we’re a long way, in my view, from being in a position to roll them back or push them out of Iraq,” Gates said on NBC’S “Meet the Press” referring to ISIS.

Gates, who served in the role under both Obama and former President George W. Bush, has repeatedly criticized Obama for ruling out ground forces in the fight against ISIS militants. He said Sunday that defeating ISIS without boots on the ground is an “unattainable objective.”

“The notion of the alternative as being what we’re doing now, and a reinvasion of Iraq, if you will, with large ground forces, is a false set of options,” he said.

“It will be very difficult to roll ISIS back without forward air controllers and spotters, without embedded trainers.” Gates added.

“What I’m talking about here is potentially a few hundred troops, not thousands or tens of thousands.”

As an international coalition fights to stamp out ISIS, Gates said the U.S. may need different goals in the fight.

“We’ve been after al Qaeda with all the resources of an American military and intelligence community for 14 years and we haven’t destroyed it,” he said.

“With respect to ISIS, our objective should be to deny them the ability to hang onto territory, because that gives them a base from which potentially to plot against us and against Western Europe,” he added.