America has said it is not ready to suspend economic sanctions against Iran because its compliance with the nuclear deal has not been verified.
White House spokesman Josh Earnest said on Friday that Iran was making important progress toward curbing its nuclear programme and that it is possible that Tehran has already completed all the necessary steps.
But Earnest said Iran will not get any sanctions relief until the International Atomic Energy Agency (IAEA) has independently verified that all steps have been completed. Washington wants to make sure Iran does not "cut any corners." he added.
US officials earlier in the week had said that "implementation day" for the Iran nuclear deal could occur within days.
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.
Meanwhile on Friday, Washington said Secretary of State John Kerry will meet with Iran's foreign minister and the European Union's foreign policy chief in Vienna on Saturday to discuss Tehran's nuclear deal.