Iran and six powers have been negotiating in Vienna since April to work out steps for Tehran and Washington to take on, respectively, nuclear activities and sanctions, for the pact to resume.
On Monday, a report from the U.N. nuclear watchdog showed that Iran had failed to explain traces of uranium found at several undeclared sites, possibly setting up a fresh diplomatic clash with the potential to derail the talks.
"There is no impasse in the Vienna talks," Iranian government spokesman Ali Rabiei told a news conference streamed live by a state-run website.
"Negotiations have reached a stage where a few key issues need to be decided, and these issues require the proper attention, perfectionism and time."
Since former U.S. President Donald Trump pulled out of the deal three years ago and reimposed sanctions on Iran, Tehran has embarked on counter-measures, including rebuilding stockpiles of enriched uranium, a potential pathway to nuclear bombs.
"It is natural that due to the complexities created by the Trump administration's numerous sanctions and Iran's measures... Many details need to be considered, but none of these obstacles are insurmountable," Rabiei added.
Also on Monday, Iran's nuclear negotiator expressed doubt that the current round of talks would be the final one.
U.S. President Joe Biden has said Washington will return to the pact if Tehran first resumes compliance with its strict limits on uranium enrichment.
