Teams from Iran and group of world powers are meeting for a third time as they try to agree on a comprehensive deal to ensure that Iran's nuclear program is peaceful.
Iranian Foreign Minister Javad Zarif and European Union foreign policy chief Catherine Ashton, the lead negotiators for the two sides, chaired an opening session Tuesday in Vienna.
They want to reach a permanent deal by the end of July, building on an interim agreement they struck last year.
U.S. Secretary of State John Kerry said Iran has the ability to produce enough material for one nuclear bomb in two months, but that it would not necessarily have the ability to deliver a weapon.
"I think it's fair to that I think it's public knowledge today that we're operating with a time-period for a so-called breakout of about two months. That's been in the public domain," he said.
Kerry said if Iran continued to produce material at that rate, instead of lengthening the timeline, then there will be consequences.
"If they make a decision to breakout, sanctions aren't going to be what make the difference. If they're overtly breaking out and breaking an agreement and starting to enrich and pursue it, they've made a huge consequential decision. And the greater likelihood is we are going to respond immediately," said Kerry.
Under the temporary pact, Iran agreed to curb its most sensitive nuclear activity, while the group that includes the United States, Britain, China, France, Russia and Germany gave Iran limited sanctions relief.
Both sides have said they want to begin drafting a permanent agreement next month.