Speaking ahead of a fifth round of talks in Vienna on rescuing that deal, Blinken was asked about Iranian reports that Washington had already agreed to lift some of the sanctions that have crippled the Iranian economy.

"We know what sanctions would need to be lifted if they're inconsistent with the nuclear agreement," he said on ABC's "This Week".

He added that more importantly, "Iran, I think, knows what it needs to do to come back into compliance on the nuclear side, and what we haven't seen is whether Iran is ready to make a decision.

"That's the test and we don't yet have an answer."

Donald Trump in 2017 walked away from the deal, saying Tehran had violated its "spirit" and remained a regional threat. His successor Joe Biden wants to revive the plan.

For that to happen, Washington must agree to lift the sanctions reinstated by Trump and Tehran must commit to follow the terms of the deal.

Once Trump walked away from the agreement, the Islamic republic started to abandon the constraints on its production of nuclear material.

European participants to the Vienna talks had expressed optimism as the latest round of negotiations wound up on Wednesday.

"We've made good progress," Enrique Mora, the European Union official who chaired the talks between Russia, China, Germany, France, Britain and Iran, said in a tweet on Wednesday.

"An agreement is shaping up."

Indirect negotiations between Washington and Tehran have been going on in the Austrian capital since early April, with the other five countries that are signatories to the deal acting as intermediaries.

Diplomats are hoping to get the US back on board before Iranian presidential elections on June 18.