"I think that most estimates that I've seen haven't changed since the last time we talked about it, which is probably at least a year, maybe more," he said at the start of a visit to Latin America and the Caribbean.
On Sunday, the US defense secretary said he believed Iran had still not obtained nuclear weapons capacity, and that progress towards such a capability was progressing slower than Tehran had anticipated.
The United States and Europe have long accused Iran of seeking to develop nuclear weapons under the guise of its civilian nuclear energy program, charges Tehran denies.
Last week, Iran announced the successful testing of third generation centrifuges capable of enriching uranium six times faster than those currently in operation at its Natanz enrichment facility.
The United States is seeking international support for tough new United Nations sanctions against Iran to punish it for continuing to enrich uranium.
