US President Donald Trump has said he could impose additional tariffs on countries that continue to trade with Iran, in an executive order signed on Friday.

The order does not specify the rate that could be imposed, but uses 25% as an example, and says it will apply to goods imported into the US from any nation that "directly or indirectly purchases, imports, or otherwise acquires any goods or services from Iran".

Trump has not directly commented on the order, but reiterated "no nuclear weapons" for Iran when speaking from Air Force One on Friday night.

It comes amid ongoing talks between senior US and Iranian officials in Oman, following several weeks of threats from both sides.

Trump threatened a 25% tariff of countries doing business with Iran earlier this year, in a post to Truth Social.

On 12 January, he wrote: "Effective immediately, any country doing business with the Islamic Republic of Iran will pay a tariff of 25% on any and all business being done with the United States of America."

At the time, no further detail was provided on how the tariffs would work in practice.

The White House said that this latest executive order reaffirmed the "ongoing national emergency with respect to Iran", and noted that the president may modify it of circumstances change.

It read: "The President is holding Iran accountable for its pursuit of nuclear capabilities, support for terrorism, ballistic missile development, and regional destabilization that endanger American security, allies, and interests."

There has been no immediate comment from Iran.

Starting in Oman on Friday, the ongoing talks are the first involving US and Iranian officials since last June, when the US bombed Iran's three main nuclear facilities.

The Iranian delegation was led by Iranian Foreign Minister Abbas Araghchi, while the US was represented by special envoy Steve Witkoff and Trump's son-in-law, Jared Kushner.

Speaking to reporters onboard Air Force One, Trump said Friday's talks were "very good", and that Iran "looks like it wants to make a deal very badly".

"If they don't make a deal, the consequences are very steep," the president said, adding that there will be another meeting early next week.

Oman's Foreign Minister Badr Albusaidi, who mediated the discussions, said they had been "useful to clarify both Iranian and American thinking and identify areas for possible progress".

In a statement to X, Araghchi described the talks to date as "a good start" and said that a "positive atmosphere prevailed". He said negotiators had now returned to their respective capitals for consultation.

Rising tensions between the US and Iran have raised fears of a conflict between the countries, with Trump declaring that Iran's Supreme Leader Ayatollah Ali Khamenei should be "very worried" as recently as Wednesday.

The president has said he would be prepared to strike the Middle Eastern nation if it refused to reach a deal on its nuclear programme.

Iran insists its nuclear programme is entirely peaceful and has repeatedly denied accusations by the US and its allies that it is seeking to develop nuclear weapons.

The US has also built up a significant military presence in the region, including what Trump describes as a "massive armada" in response to Iran's violent repression of nationwide anti-government protests last month.

Human rights groups have said that many thousands of people were killed in the crackdown, but the full scale of the bloodshed remains unclear due to internet restrictions imposed by the government since 8 January.