There are still big gaps between the U.S. and Iranian positions on what a mutual return to the 2015 nuclear deal looks like, a senior State Department official told reporters on Thursday ahead of the next round of talks in Vienna.

Why it matters: The talks are at a critical stage as key deadlines approach, after which a deal could be much harder to reach. The official said an agreement could be reached within a few weeks, but that the Iranian position will have to change significantly to make that happen.

The state of play: As Axios reported on Wednesday, one key gap is over the nuclear capabilities Iran would retain after returning to the deal — in particular what will happen to Iran's new advanced centrifuges.

• The senior official said it was clear what would be required of Iran under the terms of the 2015 deal, but confirmed that the Iranians were seeking less severe restrictions.
• The Iranians are also making "unrealistic demands" in terms of the sanctions they expect the U.S. to lift, the official said.
• "They have made demands that are beyond what we believe is required and they haven't committed to what we believe is required," the official said. Then there's the unresolved question of how to sequence the U.S. and Iranian steps.

Still, the official said the negotiations are "not rocket science," and a deal could be reached before Iran's June 18 presidential elections so long as the Iranians accept that the aim is "not reinventing the JCPOA, but complying with it."

• "This is a political decision the Iranians need to make," the senior U.S. official said.

At the same time, the official said the Biden administration is preparing for the possibility that the talks will break down without any deal.

• “If that happens, the Biden administration will deal with it and will do everything to make sure Iran doesn’t have a nuclear weapon — but we prefer reaching an understanding," the official stressed.

What's next: Another round of nuclear talks is expected to take place in Vienna on Friday, with U.S. envoy Rob Malley negotiating indirectly with the Iranians through EU meditators.

• On May 20, a temporary deal that allows the International Atomic Energy Agency to monitor some Iranian nuclear sites is due to expire. That could severely diminish the international community's visibility into Iran’s nuclear program and further complicate the path to a deal.