Return to 2015 nuclear deal remains uncertain and is not imminent, Washington says, despite optimism in recent weeks that indirect talks between Iran and US could soon produce an agreement.
US optimism that a deal to restore the 2015 agreement to limit Iran's nuclear development has soured, with the State Department now warning it was headed toward "Plan B" if Tehran doesn't budge.
"I want to be clear that an agreement is neither imminent nor is it certain," State Department spokesperson Ned Price said on Tuesday.
Just a week ago Washington officials were hopeful that an agreement that aimed to halt Iran's march toward nuclear weapons capability, after almost one year of negotiations, was within reach.
"We are close to a possible deal, but we're not there yet," Price said on March 16. "We do think the remaining issues can be bridged."
US officials said they thought Tehran would reach an agreement after Sunday's celebration of Nowruz, the Persian New Year.
But that tone suddenly changed the following day.
And on Tuesday, while refusing to say the talks had reached an impasse, Price said the United States had contingency plans if a deal could not be reached and Iran's alleged plans to develop nuclear weapons were not halted.
"The onus is on Tehran to make decisions that it might consider difficult," Price told reporters.
"In fact, we are preparing equally for scenarios with and without a mutual return to full implementation of the JCPOA," he said, referring to the formal name of the 2015 deal, the Joint Comprehensive Plan of Action.
Tough political choices
Experts said Iran had mostly stuck to the terms, but months after the US pullout, the Islamic republic began ramping up its nuclear programme with activities that would enhance its ability to build a nuclear weapon.
Last April, three months after he took office, President Joe Biden started new negotiations to revive the 2015 agreement, promising an easing of punishing sanctions in exchange for restoring JCPOA controls.
But the talks have proceeded with the knowledge that Tehran has already moved much closer to nuclear weapons "breakout," which would render the JCPOA moot.
Both sides have said in recent weeks that the other has to make tough political choices.
And Tehran is believed to be holding out for two objectives: a guarantee of some protection if the United States again pulls out from the deal and the removal of Washington's official "Foreign Terrorist Organisation" designation of the powerful Islamic Revolutionary Guard Corps (IRGC).
"We are still working through a number of difficult issues," Price said, while not confirming what the specific un-agreed points were.
"We know that there has to be a great deal of urgency, and we know that now the onus is on Tehran to make decisions," he said.