Iran could comply with last summer's nuclear deal as early as Friday or this weekend, officials said, requiring the United States and other nations to immediately suspend billions of dollars' worth of economic sanctions on the Islamic Republic.
John Kerry, the US Secretary of State, predicted on Wednesday that the achievement could be "within the next coming days".
Just hours after welcoming Iran's release of the 10 US sailors it detained when their boats wandered on to an Iranian island in the Persian Gulf, Kerry said in a speech at the National Defense University that implementation of the nuclear agreement will take place soon, without specifying a date.
Others in Washington and elsewhere said that the announcement could come within two days.
In Vienna, a senior diplomat from one of the six countries that cut the deal with Iran said that it would be formally declared as implemented "most probably Friday". Those officials spoke on condition of anonymity because they were not authorised to discuss the matter publicly.
In Tehran, Abbas Araghchi, Iran's deputy foreign minister, echoed that sentiment.
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He said that the International Atomic Energy Agency was expected to verify Iran's compliance on Friday and that Mohammad Javad Zarif, the Iranian foreign minister, and Federica Mogherini, the European Union foreign policy chief, would then announce "implementation day", according to Iran's official IRNA news agency.
"I believe the parties will perform their undertakings by Friday, Saturday and Sunday and the implementation day will be announced at that time," Araghchi said.
He said that the foreign ministers who negotiated the July accord - including Kerry, Zarif and their counterparts from Britain, China, France, Germany and Russia - may meet to deliver a joint statement. Kerry is expected to be in Europe this weekend on other business.
Under the terms of the deal, once Iran meets its obligations to curb its nuclear programme, it is to receive wide-ranging sanctions relief. For the US that means suspension of nuclear-related sanctions on Iranian oil, banking and commerce.
Iran was required to ship out most of its stockpile of enriched uranium, a material that can be used for making bombs, and take apart thousands of the centrifuges that enrich the material.
It also had to disable a heavy water reactor that would have been able to produce plutonium, another pathway to nuclear weapons. Most of the nuclear restrictions last for 10 or 15 years.
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While Iran accomplished all the biggest tasks in recent weeks, several technical hiccups remained. Officials had spoken of lingering questions related to Iran's uranium and plutonium programmes, including over the properties of certain centrifuges Iran would be permitted to maintain for research purposes.
It's not clear how quickly Iran would reap the benefits of the deal.
Tehran expects to quickly recoup some $100bn worth of its assets that have been frozen in overseas banks, a tremendous windfall for an economy with a GDP of little more than $400bn in 2014.
While the Iranians have aggressively courted foreign companies and investors in recent months, it is unclear how quickly contracts can be finalised.
Also unknown is how determined major Western firms, in particular, will be given the risk of "snapback" sanctions if Iran violates the deal and the history of US penalties against European banks that have previously conducted business with Iran.