A senior US official said Monday that time was running out for nuclear negotiations with Iran and urged Tehran to agree to direct talks to help forge a deal.

Speaking on background, the official said that Iran's nuclear programme was nearing "breakout" toward nuclear weapons capability, leaving just "weeks" to reach a deal that would put the programme on hold and ease sanctions on the country.

"I think we're at the point where some of the most critical political decisions have to be made by all sides" in the talks to revive the 2015 deal involving Iran, the United States, Britain, China, France, Germany and Russia, the official said.

"If our goal is to reach an understanding quickly... the optimal way to do that, in any negotiation, is for the parties that have the most at stake to meet directly," the official said, referring to the United States and Iran.

"That's been our view from the outset: we're prepared to meet with Iran if they're prepared to meet with us."

The official said Iran's enrichment program was very close to producing enough fissile material for a nuclear weapon.

It would be "deeply unfortunate" if the two sides did not have direct talks, "given how little time is left, given how critical... the decisions that need to be made are," the official said.

The latest round of talks in Vienna to salvage the Joint Comprehensive Plan of Action, or JCPOA, were put on pause last week, with the European Union's coordinator calling for "political decisions" to break the deadlock.

The original deal had given Tehran sanctions relief in exchange for curbs on its nuclear program.

But after the United States pulled out and reimposed sanctions under Donald Trump in 2018, Iran began dramatically stepping up nuclear activities.

Joe Biden's presidency helped relaunch the negotiations, but Washington has been taking part only indirectly, leaving the Europeans to meet the Iranians.

One week ago, Iran for the first time said it was open to direct negotiations with the United States, which quickly declared itself ready to hold talks "urgently."

The US official said Monday that not all of the details in a possible agreement had been resolved.

"But we are prepared to make those choices," the official said.

"We can see a path to a deal if those decisions are made and if it's done quickly."