Iran's foreign ministry said Sunday that talks with the United States slated for next weekend will remain "indirect" with Omani mediation, and focused solely on the nuclear issue and lifting of sanctions.

Iranian Foreign Minister Abbas Araghchi and US Middle East envoy Steve Witkoff held talks on Saturday in Muscat, marking the highest-level Iran-US nuclear negotiations since the collapse of a 2015 accord.

They agreed to meet again in seven days.

"Negotiations will continue to be indirect. Oman will remain the mediator, but we are discussing the location of future negotiations," foreign ministry spokesman Esmaeil Baqaei said in an interview with state TV.

He said the talks would only focus on "the nuclear issue and the lifting of sanctions" and that Iran "will not have any talks with the American side on any other issue."

Donald Trump in 2018 pulled out of the 2015 nuclear deal between Iran and world powers during his first term as US president.

Saturday's rare negotiations came weeks after Trump sent a letter to Iran's supreme leader Ayatollah Ali Khamenei urging nuclear talks while warning of possible military action if Iran refuses.

Iran and the US separately described Saturday's discussions as "constructive".

Iran said the talks were held indirectly with Oman's foreign minister acting as intermediary.

The negotiators, Araghchi and Witkoff, spoke directly for "a few minutes" after the talks, Tehran's foreign ministry said.

Another round of talks will be held on Saturday, April 19.

Asked about the talks, Trump told journalists aboard Air Force One: "I think they're going OK. Nothing matters until you get it done."

The process took place in a "friendly atmosphere", Omani Foreign Minister Badr Albusaidi said.