"They can have civil nuclear power. OK, we don't mind that, but... no one right now has a civil nuclear program with their entire enrichment infrastructure that can enrich to the 90-plus percent needed to get to fissile material and a nuclear weapon," Vance said at the Munich Security Conference in Washington DC.
Vance said the Trump administration has no problem with Iran having nuclear power.
"We're fine with that. But you can't have the kind of enrichment program that allows you to get to a nuclear weapon, and that's where we draw the line," he said.
The United States and Iran appear to be on a collision course over whether Tehran should be allowed to enrich uranium in any nuclear deal between the two arch-foes, potentially endangering talks headed for a fourth round this weekend.
Last month, US Secretary of State Marco Rubio said "there’s a pathway to a civil, peaceful nuclear program if they want one, but if they insist on enriching, then they will be the only country in the world that doesn’t have a 'weapons program,' quote-unquote, but is enriching. And so I think that’s problematic."