Secretary of State John Kerry spent hours locked behind closed doors with Iran’s foreign minister early Friday as both sides rushed to reach a final nuclear agreement that sources say is becoming increasingly elusive as a result of Tehran’s intransigence.

As the United States seeks to impose clear and verifiable limits on Iran’s nuclear research work ahead of a Nov. 24 deadline, the Iranians have refused to cede any ground and are publicly insisting that its “inalienable” nuclear rights must be recognized under any final deal.

Kerry and Iranian Foreign Minister Javad Zarif spent several hours Thursday night and several more on Friday morning meeting in private, according to a senior State Department official. There is no indication yet that major headway has been made between the sides.

The foreign ministers from the negotiating countries, including the United States and Iran, are all gearing up to leave Vienna today, according to one source familiar with the status of the talks. While it remains unclear at this point if the leaders will return in the coming days, some have speculated that Zarif could be conveying the parameters of a tentative deal with higher-level officials in Tehran.

Any agreement that it is reached is likely to pave the way for another extension in talks as final details continue to be hashed out, the source said.

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