Former CIA director Michael Hayden made the assertion after he arrived in Tel Aviv Monday to speak at a seminar at the Institute for National Security Studies, Haaretz reported.
"I do not underestimate the Israeli talent, but geometry and physics tell us that Iran's nuclear program would pose a difficult challenge to any military," Hayden said. "Israel's resources are more limited than those of the U.S."
He urged Israel not to take unilateral action, noting there was "no absolute certainty" all the sites of Iran's nuclear facilities were known.
Hayden said the destruction of the facilities would probably require multiple airstrikes.
Iran's capability to build a nuclear weapon probably would not be reached until 2013 or 2014, he said, so there was no need for the U.S. or Israel to make a decision until then.
The retired Air Force general has held multiple positions in the area of national security. He was director of the National Security Agency from 1999 to 2005, and was principal deputy director of national intelligence and head of the CIA under President George W. Bush.