Michael Hayden, a former CIA chief, has said it is a matter of time before ISIS attacks the West.
"An ISIS attack against the West is a question of timing not of inevitably, not of intent," Hayden said in comments reported by CNN on Monday.
Hayden said he expects ISIS to attempt an attack on targets in the United States or Europe.
"ISIS is a very powerful local organisation, and probably a reasonably powerful regional terrorist organisation," Hayden said. "But it's one that has global ambitions -- and it has the tools."
There has been a debate in the West and especially the US about how big a threat ISIS is to them.
That debate came to the forefront last week after the Sunni militant group beheaded American journalist James Foley in retaliation for airstrikes -- an act that Michael Morell, the former deputy director of the CIA, called the group's "first terrorist attack against the United States."
"The IS will come sooner rather than later," Hayden said.
"An American interest in the region right now is at risk. We have kind of underestimated our opponents in the past, like in 9/11 and with Al-Qaeda," Hayden admitted.
Hayden said that American airstrikes inside Syria, in addition to those being conducted in Iraq, would make a difference in the fight against ISIS. Airstrikes in Iraq come at the request of that government; Syria, however, is in the midst of a civil war in which almost 200,000 people have been killed. The United States has called for its President, Bashar Al-Assad, to step down.
"It's not just about defence; it's not just about keeping the right people off of aircraft," Hayden said. "It's about offense. It's about disabling ISIS. It's about making them more worried, more consumed with protecting their own survivability rather than yours or mine," Hayden asserted.