If Iran insists that all sanctions be removed in order to reach a deal it will lead to an impasse, the official added. Their discussions were focused on defining "a common set of steps" that Iran and the United States could take, and "if and when" they get closer to a common understanding on those steps, they will likely discuss sequencing.
The amount of new enriched uranium is relatively small but threatens to derail talks being held to revive the deal.
The remaining parties have formed two expert-level working groups whose job is to draw up lists of sanctions that the United States will lift and of nuclear restrictions Iran will implement.
US Democratic President Joe Biden had vowed to reverse Trump's Iran policy and return to the JCPOA.
"Questions still remained about whether the seriousness of goal - in terms of coming back into compliance that the U.S. showed - will be reciprocated by Iran".More news: Thirty-Five New COVID-19 Cases Reported Locally
The official also noted that the United States delegation in Vienna brought up the issue of Americans detained and missing in Iran.
Former President Donald Trump withdrew from the deal in 2018.
Police officers stay in front of the "Grand Hotel Wien" in Vienna, Austria, April 9, 2021, where closed-door nuclear talks with Iran take place.
The multilateral agreement, known as the Joint Comprehensive Plan of Action (JCPOA), saw Iran scale back its nuclear programme in exchange for lifting sanctions against its economy. European intermediaries relayed messages to the Americans from the talks.
The deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), covers only nuclear sanctions and not U.S. measures taken in response to human rights or other concerns, the official said.More news: State trooper among 6 shot in Texas shootings
The JCPOA called for Iran to freeze nuclear activities that could be weaponized in return for sanctions relief from world powers.
"Otherwise, there will be no reason to continue negotiations", he said.
The indirect talks with Iran would resume in Vienna next week, a USA official has said, noting that the meeting held this week through the European Union, China and Russian Federation made some progress.
The EU diplomat leading the talks, Enrique Mora, said that the meetings had been "constructive and results oriented".
Speaking in the Austrian capital on Wednesday, University of Vienna politics professor Heinz Gärtner said the USA feels a sense of urgency to get an agreement before Iran holds a June presidential election whose victor could take a tougher stance toward the US, albeit only with the permission of Supreme Leader Ayatollah Ali Khamenei, who has the final say in key decision-making.More news: Biden Nominating Gabby Giffords’ Gun Control Affiliate to Head ATF
Moscow's ambassador to the United Nations in Vienna Mikhail Ulyanov also said that the participants "noted with satisfaction the initial progress made" and wanted to "maintain the positive momentum".