Big gaps between the U.S. and Iran over the measures needed to roll back and limit the Iranian nuclear program are stalling the Vienna talks, European diplomats and former U.S. officials briefed on the issue tell me.

What's happening: The Biden administration has said any deal to restore the 2015 nuclear accord must include a return by Iran to full compliance with its previous commitments. But that's complicated by the fact that Iran's nuclear program has advanced since 2015.

• The U.S. and European signatories on the deal agree that Iran's "breakout time" — the time needed to produce enough enriched uranium for a bomb — must be at least a year. They also have a common position on what it would take to get there, a European diplomat tells me.
• But Iran's position is much different. One key disagreement is over what will happen to the new, more sophisticated centrifuges Iran has installed that allow Tehran to enrich uranium much more quickly, the diplomat says.
• Any new deal will have to determine whether Iran can still use those centrifuges and, if not, whether they would need to be taken out of the country or simply disconnected and stored in Iran.

Between the lines: Much of the discussion around the nuclear talks has focused on disagreements over which sanctions the U.S. would have to lift.

• There has been substantive progress on that front in Vienna, but a separate working group handling the nuclear side of the equation has made almost no headway.

The state of play: The latest round of talks over the weekend did not produce any major progress, and were placed on hold while many attended a G7 meeting. They are expected to resume Friday in Vienna.

What to watch: Two major deadlines loom.

• On May 20, a temporary deal that allows the International Atomic Energy Agency to monitor some Iranian nuclear sites expires. The Iranians are threatening to shut down IAEA cameras in those sites and thus severely diminish the international community's visibility into Iran’s nuclear program.
• On June 18, Iran will hold presidential elections that could have a dramatic influence on the nuclear talks.