"We want to guarantee that the sanctions are effectively lifted," foreign ministry spokesman Esmaeil Baqaei told a news conference in Tehran.

"So far, the American side has not wanted to clarify this issue," he added.

His remarks come a day after a report by the UN agency showed Iran has stepped up production of uranium enriched up to 60 percent -- close to the roughly 90 percent level needed for atomic weapons.

The US envoy in the nuclear talks said last month that the administration of President Donald Trump would oppose any enrichment.

"An enrichment programme can never exist in the state of Iran ever again. That's our red line. No enrichment," Steve Witkoff told Breitbart News.

Iran has vowed to keep enriching uranium "with or without a deal" on its nuclear programme.

The United States has sent Iran a proposal for a nuclear deal that the White House called "acceptable" and in its "best interest" to accept, US media reported on Saturday.

The New York Times, citing officials familiar with the diplomatic exchanges, said the proposal calls on Iran to stop all enrichment and suggests creating a regional grouping to produce nuclear power.

Iran has held five rounds of talks with the United States in search of a new agreement to replace the deal with major powers that Trump abandoned during his first term in 2018.