Iran says its nuclear programme is for peaceful purposes
Iran faces "significant" new sanctions over its nuclear programme, according to US Defence Secretary Robert Gates.
And he reiterated that all options - including possible military action - should stay on the table.
Mr Gates was speaking to US troops during a visit to northern Iraq.
His comments come a day after the US, Britain and France warned that Iran faced tougher sanctions unless it immediately complied with UN Security Council resolutions.
Mr Gates said: "I think that you are going to see some significant additional sanctions imposed by the international community, assuming the Iranians don't change course."
But despite warning that possible military action should stay on the table, he said it would "only buy some time, maybe two or three years".
Iran is already subject to UN sanctions over its nuclear programme, which the West suspects is for military purposes.
Any move towards a new round of sanctions is expected to involve long and arduous negotiations with reluctant security council members, such as China, says the BBC's Barbara Plett at the UN.
There is also the question about whether or not agreement could be won on significantly tougher restrictions, she adds.
Iran has insisted its nuclear programme is for purely peaceful purposes and has warned that further sanctions will be ineffective.
It has not signed up to a six-nation compromise plan over its controversial uranium enrichment programme.
Russia, China, the US, UK, France and Germany have suggested uranium enrichment for civilian nuclear energy could be regulated if Iran handed over its uranium to Russia to manage the process.