The issue has come into focus in recent weeks, with Iran staunchly defending its right to enrich uranium as part of a civilian nuclear programme, while the United States wants it to stop.
The negotiations, which began in April, are the highest-level contact between the two sides since the United States left a landmark 2015 nuclear accord during US President Donald Trump's first term.
US Middle East envoy Steve Witkoff, who is leading the talks for Washington, said the United States "could not authorise even one percent" of enrichment by Iran.
- 'Totally false' -
On Monday, Iran ruled out suspending its uranium enrichment.
"This information is a figment of the imagination and totally false," said foreign ministry spokesman Esmaeil Baqaei, asked about the possibility during a press briefing in Tehran.
Iran insists it has the right to a civilian nuclear programme, including for energy, and considers the US demand a red line that violates the Nuclear Non-Proliferation Treaty, to which it is a signatory.
MNA