The window is still open to revive the 2015 Iran nuclear deal but Tehran has yet to indicate whether it is willing to resume talks in Vienna or whether it would do so on the basis of where they left off in June, a senior US official said on Thursday.

The official told reporters on condition of anonymity that Washington's patience would not last forever but declined to set a deadline, saying this depended on technical progress in Iran's nuclear program and a wider judgment by the United States and its partners on whether Iran was willing to revive the deal.

"We're still interested. We still want to come back to the table," the senior US State Department official said in a telephone briefing. "The window of opportunity is open. It won't be open forever if Iran takes a different course."

Under the 2015 deal, Iran curbed its uranium enrichment program, a possible pathway to nuclear arms, in return for the lifting of economic sanctions. Former President Donald Trump quit the deal three years ago and re-imposed harsh sanctions on Iran's oil and financial sectors that have crippled its economy, prompting Iran to take steps to violate its nuclear limits.

The US official declined to say what the United States might do if Iran refuses to return to negotiations, or if a resumption of the original deal proves impossible. Such U. contingency planning is often referred to as "Plan B."

"The 'Plan B' that we're concerned about is the one that Iran may be contemplating, where they want to continue to build their nuclear program and not be seriously engaged in talks to return to the JCPOA," he said, in a reference to the deal's formal name, the Joint Comprehensive Plan of Action.