"The United States team put forward a very serious idea and demonstrated a seriousness of purpose on coming back into compliance if Iran comes back into compliance," the official told reporters as talks broke for the weekend.

But the official said the United States was waiting for its efforts to be "reciprocated" by Iran.

"We saw some signs of it but certainly not enough. There's still question marks about whether Iran has the willingness to... take the pragmatic approach that the United States has taken to come back into compliance with its obligations under the deal," he said.

President Joe Biden supports a return to the 2015 agreement trashed by his predecessor Donald Trump under which Iran drastically scaled back nuclear work in return for promises of sanctions relief.

Iran has demanded that the United States first lift all sanctions imposed by Trump, which include a sweeping unilateral ban on its oil exports, before it falls back in line with obligations it suspended.

The US official indicated that the major stumbling block in the initial talks was not the order of compliance but rather which sanctions were under discussion as Iran is demanding an end to all US restrictions.

Iran's position is "not consistent with the deal itself because under the deal the US retains the right to impose sanctions for non-nuclear reasons, whether it's terrorism or human rights violations or interference with our elections," the official said.

"All sanctions that are inconsistent with the JCPOA and are inconsistent with the benefits that Iran expects from the JCPOA we are prepared to lift. That doesn't mean all of them because there are some that are legitimate sanctions," he said, using the acronym for the accord's official name.

Iran refused to meet directly with US negotiator Rob Malley during the talks led by the European Union, whose envoys shuttled between the two sides in different hotels. Talks are set to resume in the same format next week.