"We should know in pretty short order if the Iranians are going... to negotiate in good faith," said the spokesman, Ned Price, warning that "the runway is getting very, very short for negotiations."

The European Union, which is coordinating the indirect talks between Washington and Tehran, confirmed they would resume Thursday in Vienna after a break of a few days.

US envoy Rob Malley "will plan to join the talks over the weekend," Price said.

The talks began in April but were suspended in June due to the election of a new Iranian president, only to resume last week.

After a week of negotiations, Americans and Europeans alike accused the Iranians of having backtracked since the spring.

Washington has warned it will not let Tehran block negotiations for much longer while developing its nuclear program at the same time, but has not yet laid out an ultimatum.

The next few days look set to see a last-chance diplomatic push, although it appears ever more unlikely that the talks will lead to any breakthrough.

"I don't think you will see a long lag between the resumption of this round and when the United States and our allies and partners are in a position to judge whether the Iranians have returned... with a willingness to engage in substantive negotiations," Price told reporters.

"It will not always be in our interest to seek a return to the JCPOA," he said, using the formal title of the landmark 2015 accord that aimed to curb Iran's nuclear program to ensure it could not develop an atomic weapon, in exchange for sanctions relief for Tehran.

The Joint Comprehensive Plan of Action was initially agreed between Iran and Britain, China, France, Germany, Russia and the United States.

But it began unravelling in 2018 when then US president Donald Trump pulled out and reimposed sanctions, prompting Iran to start exceeding limits on its nuclear program the following year.

Iran has always insisted that its nuclear program is peaceful.