Major hurdle still exists as Iran nuclear talks reach finish line

The backstory: Over the past several years, evidence has surfaced suggesting Iran might have undeclared nuclear material in secret sites around the country.

• Samples taken by UN inspectors who visited the sites only raised more questions, but Iran has declined to answer them.
• Israel claims that's evidence that Iran continued to hide parts of its nuclear program after joining the 2015 deal.

Driving the news: Iranian chief negotiator Ali Bagheri returned to Vienna earlier this week after consultations in Tehran for what may be the final days of negotiations.

• U.S. officials say that although the parties are close to a deal, differences remain, including the IAEA probe issue and Iran's demand that the U.S. remove the Islamic Revolutionary Guard Corps from its terror blacklist.
• Washington continues to insist that if a deal isn't reached within a matter of days, there will be no point in continuing negotiations.

What they're saying: A senior Biden administration official told me that it was impossible for anyone other than IAEA Director-General Rafael Grossi to determine that open investigations had been resolved.

• Grossi said Wednesday that his agency will never stop a process that it launched for "political reasons."
• U.K. negotiator Stephanie Al-Qaq tweeted that the European powers will always reject any attempt to compromise IAEA independence.