Tehran and Washington maintain strong differences on how to revive the deal that former President Donald Trump abandoned, with Iran insisting on full and guaranteed sanctions removal in order for Iran to scale back its nuclear activity, while Washington has ruled out any “unilateral gestures.”

State Department spokesman Ned Price said on Monday he expected the talks to be “difficult” and wasn’t forecasting any breakthroughs, adding that Robert Malley, the U.S. special envoy for Iran, would lead the talks on behalf of the Biden administration.

Read more: Iran Rules Out Any Talks With U.S. in Vienna Unless Sanctions Go

Earlier in the day, Iran’s Foreign Ministry spokesman, Saeed Khatibzadeh, said it was up to the other powers that are still in the deal to ensure that Washington “corrects” its path and provides the guaranteed and verifiable removal of bruising penalties on Iran’s economy imposed by Trump.

Khatibzadeh said Iran was not interested in a “step by step” approach, reiterating comments made on Sunday by his country’s lead negotiator, Deputy Foreign Minister Abbas Araghchi, who has already arrived in Vienna ahead of the talks.

Iran breached some of the limits on its nuclear program more than a year after Trump abandoned the deal that was championed by his predecessor, Barack Obama. Tehran has since repeatedly rebuffed direct talks with the Biden administration on reviving the pact.

Khatibzadeh said that the “window of opportunity” to break the deadlock with the U.S. over how to salvage the deal won’t close if the talks don’t deliver a breakthrough, but warned it “won’t remain open forever.”