The indirect talks between the US and Iran over respective steps to revive the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), were just preliminary steps but "productive and businesslike", a senior state department official told reporters in a briefing on Friday.
"At the same time, a question still remains about whether the seriousness of purpose and the intent of coming back into compliance that the US showed will be reciprocated by Iran," the official said.
"We saw some signs of but certainly not enough."
The official reiterated that Washington is prepared to lift those sanctions that are inconsistent with the nuclear deal, reports Xinhua news agency.
"Without getting into specifics, I would say that that's our position, 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," the official said, adding but "that doesn't mean all of them because there are some that are legitimate sanctions".
Meanwhile, Iran's Deputy Foreign Minister Abbas Araqchi also said on Friday that Tehran will not stop parts of its current nuclear activities until the US lifts sanctions and returns to the deal.
The country will not halt or even reduce the pace of its current nuclear activities in particular its uranium enrichment, including the 20 per cent enrichment, Araqchi was quoted as saying by the official Islamic Republic News Agency.
This will continue until an accord is reached under which the US will be obliged to lift all of its sanctions against Iran, he said, adding that the American sanctions should be lifted in one step.
Araqchi, who also represents Iran in the ongoing talks with the remaining parties of the nuclear deal, noted that there are signs that Americans are considering the removal of all sanctions.
On Friday, the diplomatic representatives of Iran, the UK, China, France, Russia and Germany, resumed talks in Vienna with the lifting of sanctions on Iran and nuclear implementation measures on top of the agenda.
On Tuesday, the senior diplomats held the first round of their meetings to discuss the current standoff in the JCPOA implementation in Vienna.
Under the deal, Tehran agreed in 2015 to roll back parts of its nuclear program in exchange for decreased economic sanctions.
Washington under former President Donald Trump withdrew from the JCPOA in 2018 and tightened sanctions on Iran.
The US and Iran are at a stalemate over reviving the nuclear deal.
President Joe Biden's administration said that if Iran returns to full compliance with the JCPOA, the US would do the same.
But Iran insisted that its compliance would only take place once the sanctions are removed.