"This is a terrorist problem that affects us, and we have to take a more forward-leaning posture," said Michele Flournoy, who served as Defense Department under secretary of policy during President Barack Obama's first term, on CNN's "State of the Union" on Sunday.
"The truth is, ISIS is a threat not just in Iraq and Syria, it is a threat to us, particularly given the flow of foreign fighters," she said.
Her comments come in the wake of an ISIS victory over Iraqi troops in the city of Ramadi last week. The development infuriated U.S. military members who had fought to take the city and led to a new round of concerns that the Iraqi army isn't strong enough to handle the threat of ISIS.
Senate Armed Services Committee Chairman John McCain, R-Arizona, laid into the Obama administration on CBS's "Face the Nation" on Sunday, blaming the White House for a deteriorating security situation in Iraq. He called on the United States to put several thousand more troops on the ground in Iraq.
"We need to have a strategy," McCain said. "There is no strategy. And anybody that says that there is I'd like to hear what it is. Because it certainly isn't apparent now."
Defense Secretary Ash Carter told CNN's Barbara Starr on Sunday Iraqi troops had shown little will to fight during the encounter.
"What apparently happened was that the Iraqi forces just showed no will to fight," Carter said.
Flournoy said the United States should turn its focus to arming Sunni tribes and Kurds and called for "more resources to the provinces" where they can be more effective.
"The people with the political will to fight are the Sunni tribes, and we have got to ensure that our efforts are directed at helping them," she said.
Rep. Tulsi Gabbard, a Hawaii Democrat, agreed, telling CNN's Jim Acosta that "clearly ISIS has gained momentum here."
She said the persecution of Sunnis by the Iraqi government's Shia military has meant that for Sunnis in Iraq, there is at times "no place else to turn to protect themselves and their families except ISIS."
Rep. Adam Kinzinger, an Illinois Republican who like Gabbard is a military veteran, said Obama's public resistance to a ground war in Iraq has given the appearance that he believes "the existence of boots on the ground is worse than the existence of ISIS."
"We're not really engaged in this fight," he said. "At some point, we're going to have to understand that the goal is the destruction of ISIS."