After nearly a year of negotiations, "the onus is on Tehran to make decisions that it might consider difficult," State Department Spokesman Ned Price said.

"There are a number of difficult issues that we are still trying to work through," he added.

"An agreement of this sort is neither imminent nor is it certain and so, that is precisely why for the better part of the year, we have been preparing for either contingency," Price said.

Without going into details, Price said Washington had long been discussing "alternatives" with its partners in the Middle East and Europe.

He reiterated that US President Joe Biden's commitment to prevent Iran from acquiring a nuclear bomb would be upheld, with or without an agreement with Tehran.