"Participants will go back to capital(s) for consultations and instructions to come back next week. Political decisions are needed now," Mora tweeted.

The deal -- sealed in 2015 by Iran, the United States, Britain, China, France, Germany and Russia -- gave Tehran sanctions relief in exchange for curbs on its nuclear programme.

However after the US pulled out of the accord and reimposed sanctions in 2018 under then-president Donald Trump, Iran began dramatically stepping up its nuclear activities.

Talks aiming to rescue the faltering accord began in the Austrian capital in April and -- after a five-month suspension -- resumed in November.

The EU has played the role of mediator, while Washington has been taking part only indirectly in the negotiations.

But on Monday Iran for the first time said it was open to direct negotiations with the US, which quickly declared itself ready to hold talks "urgently".

The potential turning point came after US Secretary of State Antony Blinken said in mid-January that it was a "decisive moment" in the negotiations, warning there were only "a few weeks left" to save a deal.