Iran and world powers are engaged in talks in Vienna to rescue the 2015 nuclear deal after former US president Donald Trump walked away from it in 2018 and reimposed crippling sanctions on Tehran.

Trump's successor Joe Biden has signalled his willingness to revive the plan.

For this to happen, the US would need to return to the accord and lift the sanctions reinstated by Trump while Tehran would have to re-commit to full compliance with nuclear obligations it progressively withdrew from since 2019.

"We have had very significant progress and still think that an agreement is within reach," Iranian foreign ministry spokesman Saeed Khatibzadeh told reporters Monday.

The Vienna talks "can easily lead to results if a political decision is made in Washington", he said.

That decision would include "distancing themselves from the rhetoric, attitude and behaviour" of Trump who had waged a campaign of "maximum pressure" on Tehran, he said.

On Sunday, ahead of a fifth round of talks, US Secretary of State Antony Blinken said it remains unclear whether Iran is "ready and willing" to take the steps to return to the deal's nuclear obligations.

"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," he said.

"That's the test and we don't yet have an answer," he added on the ABC television news show "This Week" .

Iranian Foreign Minister Mohammad Javad Zarif reacted to Blinken's comments by tweeting that the lifting of sanctions reimposed by Trump "is a legal & moral obligation, NOT negotiating leverage".

"Trump's legacy is past its expiration date. Drop it," he added.

Washington and Tehran have been taking part in indirect negotiations in the Austrian capital since early April, with the other five countries still party to the deal -- Britain, China, France, Germany and Russia -- acting as intermediaries.

On Wednesday negotiators said an agreement was "shaping up" to bring the US back into the deal, citing headway in efforts to break the impasse.

But Khatibzadeh warned the talks could drag on if the US clings to the "failed legacy" of Trump's administration.