The US and Iran have agreed to hold nuclear talks in Oman on Friday, as President Donald Trump issued a blunt warning to Supreme Leader Ayatollah Ali Khamenei.

Iranian Foreign Minister Abbas Araghchi said that the meeting would start at 10:00 (06:00 GMT) in Muscat. US officials also confirmed it would happen there.

The talks had appeared to be in jeopardy, with the two countries at odds over the location and parameters.

Trump has built up US forces in the region and threatened military action if Iran does not agree a deal on its nuclear programme and stop killing protesters. Asked whether Khamenei should be worried, he told NBC News on Wednesday: "I would say he should be very worried."

"He should be. As you know, they're negotiating with us," he added.

Khamenei warned the US on Sunday that any attack on Iran would spark a "regional war".

An Arab diplomat told the BBC's US partner, CBS News, that the negotiations between Tehran and Washington were never officially called off but had been in flux on Wednesday morning.

Axios said the administration agreed to the request "to be respectful" to its allies but that it was "very sceptical" about the prospects of success.

US Secretary of State Marco Rubio earlier told a news conference that special envoy Steve Witkoff had been preparing to meet Iranian officials in Turkey along with representatives of other regional powers, when they received "conflicting reports" about Iran's participation.

Rubio also insisted that for the talks to "lead to something meaningful" they could not focus solely on the Iranian nuclear programme, as Iranian officials have demanded.

"They will have to include certain things, and that includes the range of their ballistic missiles, that includes their sponsorship of terrorist organisations across the region, that includes their nuclear programme, and that includes the treatment of their own people," he said.

"President Trump said, 'no nuclear weapons,' and we fully agree with that. That could be a very good deal. Of course, in return we expect sanction lifting. So that deal is possible. Let's not talk about impossible things," Araghchi replied.

Iran has insisted its nuclear programme is entirely peaceful and denied that it is seeking to develop nuclear weapons.

However, Trump said in the NBC interview that Iran was "going to have a nuclear weapon within one month" before he ordered US air and missile strikes on three key Iranian nuclear facilities during a 12-day war between Israel and Iran last June.

The Israeli military also targeted Iran's nuclear facilities and nuclear scientists during the war, as well as its military commanders and missile arsenal.

Trump said the strikes "obliterated" Iran's uranium enrichment capacity, but that Iranian officials "were thinking about starting a new site in a different part of the country".

"We found out about it. I said, 'You do that, we're going to do... very bad things to you.'"

Trump also told Iranian protesters that "we've had their back", following the brutal crackdown by Iranian security forces on the anti-government unrest last month.

The protests were sparked by anger over the collapse of the Iranian currency and soaring cost of living, but they quickly widened into demands for political change.

The full scale of the bloodshed resulting from the crackdown is still not known because of an internet shutdown imposed by the government since it escalated on 8 January.

However, the US-based Human Rights Activists News Agency (HRANA) has said it has confirmed the killing of 6,445 protesters, 164 children, 214 people associated with the government, and 60 bystanders. It is also investigating reports of another 11,280 deaths.

Iranian authorities have acknowledged that at least 3,117 people were killed, but said the majority were members of the security forces or bystanders killed by "rioters".

Khamenei described the unrest as "sedition" orchestrated by the US and Israel.