The US has said Iran must decide between confrontation and co-operation in the dispute over its nuclear programme.
The warning came after talks in Geneva, where envoys from the US, the EU and the UN asked Iran to suspend its uranium enrichment in return for a pledge not to introduce new sanctions.
The meeting was the first time US and Iranian officials have held face-to-face talks on the nuclear issue.
Iran gave no guarantees it would halt its activities. Iran has been given two weeks to provide an answer.
