"We're going to know very, very quickly, I think in the next day or two, whether Iran is serious or not," US Secretary of State Antony Blinken told reporters in Stockholm on the sidelines of a meeting of the Organization for Security and Co-operation in Europe (OSCE).

"In the very near future, the next day or so, we will be in a position to judge whether Iran actually intends now to engage in good faith."

But he cautioned: "I have to tell you, recent moves, recent rhetoric, don't give us a lot of cause for optimism."

In a phone call with Blinken on Thursday, Israeli Prime Minister Naftali Bennett had called for the "immediate cessation" of talks, which resumed on Monday.

Blinken declined to comment directly on the request.

"But even though the hour is getting very late, it is not too late for Iran to reverse course," Blinken said.

"What Iran can't do is sustain the status quo of building their nuclear programme while dragging their feet on talks. That... will not happen."

"That's also not our view alone. It's very clearly the view of our European partners. I have to say I had a good conversation as well" with Russian Foreign Minister Sergei Lavrov.

"I think Russia shares our basic perspective on this."

The 2015 agreement -- known as the Joint Comprehensive Plan of Action or JCPOA -- offered Iran a lifting of economic sanctions in return for strict curbs on its nuclear activities.

The goal was to make it practically impossible for Iran to build an atomic bomb, while allowing it to pursue a civilian nuclear programme.

But the deal started to unravel in 2018 when then US president Donald Trump pulled out and began imposing sanctions on the Islamic republic.

In turn, Iran, which denies it wants to acquire a nuclear arsenal, has gradually abandoned its commitments since 2019.

- 'Up to the West' -

US President Joe Biden has said he is willing to return to the deal as long as Iran also resumes the original terms.

Iran said on Thursday it had handed European powers two draft proposals to try to revive the JCPOA.

Lead negotiator Ali Bagheri told Iranian state television the proposals -- submitted on Wednesday, the third day of the talks in Vienna -- concerned two main issues facing the pact: the lifting of sanctions and Iran's nuclear commitments

"The first document sums up the Islamic republic's point of view concerning the lifting of sanctions, while the second is about Iran's nuclear actions," Bagheri told IRIB TV.

"Now the other side must examine these documents and prepare itself to hold negotiations with Iran based on these documents," said Bagheri.

Bagheri, echoing Foreign Minister Hossein Amir-Abdollahian, said Iran was in Vienna to resume talks but said it was up to the West.

"We have told the other side that we are in Vienna to pursue the talks... If they are ready to pursue the talks, we agree to pursue them," he told journalists in Vienna.

He said a timetable for the resumption of negotiations would be set up on Thursday.