In a statement, foreign ministry spokesman Esmaeil Baqaei said Washington's policy of imposing sanctions on the Iranian people was a "clear contradiction with the United States' demand for dialogue and negotiation and indicates America's lack of goodwill and seriousness in this regard".

On Tuesday, the US Department of the Treasury imposed sanctions on an Iranian shipping network and an individual named Asadoollah Emamjomeh, who Washington says is the network's owner.

It said in a statement the network was "collectively responsible for shipping hundreds of millions of dollars' worth of Iranian LPG and crude oil to foreign markets".

"Emamjomeh and his network sought to export thousands of shipments of LPG -- including from the United States -- to evade US sanctions and generate revenue for Iran," said Treasury Secretary Scott Bessent.

"The United States remains committed to holding accountable those who seek to provide the Iranian regime with the funding it needs to further its destabilising activities in the region and around the world."

The sanctions came after Tehran and Washington held two rounds of indirect nuclear talks on consecutive Saturdays in Muscat and Rome, starting on April 12.

Since returning to office in January, US President Donald Trump has reimposed sweeping sanctions under his policy of "maximum pressure" against Tehran.

In March, he sent a letter to Iran's supreme leader Ayatollah Ali Khamenei calling for talks but also warning of possible military action if they failed to produce a deal.

Oman said the third round of talks, set for Saturday, April 26, would again be held in Muscat.

On Tuesday, Iran announced that a technical expert-level nuclear meeting between the two countries will also be held on Saturday.

"The expert and high-level indirect talks in Oman will not be held simultaneously," state TV reported on Wednesday.

"Iranian and American experts will first hold their indirect talks and convey the results of the talks to the high-level officials, who will then start their discussions" the report said.

Iranian Foreign Minister Abbas Araghchi and US Middle East envoy Steve Witkoff are leading the negotiations, with Oman mediating.

After last Saturday's Rome talks, Araghchi said the meeting was "good" and that the negotiations were "moving forward".

On Monday, Trump said Washington had "very good meetings" on Iran.

Araghchi was in Beijing on Wednesday for talks ahead of Saturday's meeting.

China is Iran's largest commercial partner and the main buyer of its oil.

Western countries including the United States have long accused Iran of seeking to acquire nuclear weapons -- an allegation Tehran has consistently denied, insisting that its programme is for peaceful civilian purposes.