01:47 - Source: CNN
Kerry: Substantial progress made with Iran
Lausanne, Switzerland CNN —  

The U.S. put the onus on Iran to make compromises in order to reach agreement, as talks over Tehran’s nuclear program reached a critical point Friday.

The foreign ministers of Britain, France and Russia prepared to travel to Switzerland this weekend to lock in a political framework pact that places curbs on Iran’s nuclear program ahead of Tuesday’s deadline.

“Yesterday’s and today’s talks have been tough and very serious,” a senior state department official said. “We’re at that point in the negotiations where we really need to see decisions being made. We will test whether that is truly possible over the next several days.”

But Iranians pushed back on the notion that it was up to them to bridge the last gaps that remain between them and the world powers with whom they are negotiating.

“We have come here with very clear decisions that we have made, and we have expectations that our partners also would be deciding likewise on the key issues,” said Hamid Baeedinejad, a senior member of the Iranian delegation. “Now basically it is out partners that should makes those tough decisions that are necessary for concluding this part of the negotiations.”

READ: 5 things you need to know about the Iran talks

Still, both sides indicated they were optimistic about hammering out an agreement this week.

“We are working hard,” Secretary of State John Kerry told CNN as he was walking into the Beau Rivage hotel where the talks are being held. “We are making progress but we are not there yet. “

“Things are going, it’s tough,” Iranian Foreign Minister Mohammad Javad Zarif said after meeting with Kerry for about 90 minutes. “We are trying to move forward. We are doing our best and we hope for the best.”

The pair also met for two hours Thursday afternoon.

The talks have been held under tight secrecy as the negotiators try to resolve remaining sticking points centering around the scope of advanced nuclear research Iran could continue to do while a deal is in effect as well as the pace of sanctions relief.

In the meantime, the U.S. continues to consult with its partners. President Barack Obama and German Chancellor Angela Merkel talked by phone Friday, and agreed that “Iran must make the decisions necessary to resolve the remaining issues,” according to a statement.

Iranian President Hassan Rouhani engaged in his own public diplomacy on Thursday, taking to Twitter to write about a letter he sent to Obama and the leaders of the five other countries engaged in the talks – Britain, France, Germany, Russia and China – urging them to overcome differences in order to reach an agreement by the month’s end.

He repeated his refrain that Iran will only accept a deal if the world powers agree to drop sanctions immediately once a nuclear deal is in effect.

“Lifting all unjust #sanctions main step to reach a deal,” he wrote on Twitter.

READ: Iranian President: Nuclear deal within reach

Rouhani also spoke with world leaders – French President Francois Hollande, British Prime Minister David Cameron and Russian President Vladmir Putin.

“The fact that President Rouhani reached out to the heads of state of the P5+1 is hopefully a sign that Iran is ready to make some of the tough decisions it will have to as part of a comprehensive agreement,” the senior State Department official said.

The negotiations include capping the amount of centrifuges Iran can operate to enrich uranium, as well as reconfiguring its heavy water reactor at Arak to produce less plutonium than first envisioned – eliminating another pathway to a bomb.

A major source of disagreement – what to do with Iran’s underground Fordo facility – may also be on a path to resolution.

The parties are discussing a proposal to run several hundred centrifuges at its Fordo site, but it would not be allowed to enrich uranium, according to a senior administration official and a Western diplomat. The centrifuges would only be used for non-nuclear purposes and fed materials for medical research.

The possible compromise on a major sticking point of the talks underscored international negotiators’ optimism that a framework deal would by struck by next week’s deadline.

But even if a pact is reached, it is unclear what form it would take. A comprehensive deal including technical annexes is supposed to be concluded by June 30.

The U.S. wants the framework to be a formal written document that quantifies Iran’s commitments to submit to Congress in an effort to prevent lawmakers from imposing additional sanctions against Tehran, which could prompt Iran to withdraw from the talks.

But U.S. and Western diplomats say Zarif is reluctant to sign an initial document out of concern revealing too many details could give an opportunity to hardliners who may try to scuttle a deal. Iran, he has said, is looking simply for an “understanding” of what has been agreed to before a formal accord is reached.

“We do not know what form this will take if we can get there at the end of March,” a senior State Department official said. “The goal is to get as many specifics nailed down both with the Iranians and whatever we can say publicly.”