"The reality is, there is no military option that does anything more than buy time," Gates told CNN's John King in an interview set to air at 9 a.m. Sunday. "The estimates are one to three years or so.
"And the only way you end up not having a nuclear-capable Iran is for the Iranian government to decide that their security is diminished by having those weapons, as opposed to strengthened. And so I think, as I say, while you don't take options off the table, I think there's still room left for diplomacy."
The existence of a second uranium enrichment facility in Iran came to light Friday, prompting President Barack Obama and the leaders of Britain and France to publicly chide the Islamic republic and threaten further sanctions.
But the United States has actually known about the unfinished site since the Bush administration, according to senior U.S. officials who declined to be identified because of the sensitive nature of the negotiations.
It wasn't until Monday that Iran wrote a letter to the International Atomic Energy Agency, revealing the existence of the underground facility on a military base near the holy city of Qom, the IAEA said. Qom is about 100 miles southwest of Tehran.
"The Iranians are in a very bad spot now because of this deception, in terms of all of the great powers," Gates said. "And there obviously is the opportunity for severe additional sanctions. And I think we have the time to make that work."