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 yet seen is whether Iran is ready and willing 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, and Blinken has been working with the other signatories toward that end.

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

- 'Even greater impunity' -

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

The Trump administration's "maximum pressure" campaign against Iran, Blinken said, "clearly did not get the result that we all seek."

While some have argued that Tehran's support of extremist groups and its arms proliferation mean it cannot be trusted to observe a nuclear pact, Blinken argued rather that "an Iran with a nuclear weapon, or with the capability to build one in very short order, is going to act with even greater impunity in those areas."

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.

Meantime, the International Atomic Energy Agency on Sunday said it was delaying until Monday a Vienna news conference by its director general, Rafael Mariano Grossi.

It said Grossi was "continuing consultations with Tehran regarding the technical understanding between Iran and the UN agency."

Tehran in late February limited the IAEA's access to the nuclear sites it has been monitoring, but allowed some inspections to continue while the Vienna talks play out.