An Iranian official told Reuters the talks had not officially started, although Iran's demands had been conveyed to the US via Oman.
The official said indirect negotiations "possibly" would begin after a meeting between the top US negotiator and Oman's foreign minister. Previous Iran-US talks have adopted a shuttle diplomacy approach.
While both sides have indicated readiness to revive diplomacy over Tehran's long-running nuclear row with the West, Washington wants to expand the talks to cover Iran's ballistic missiles, support for armed groups around the region and "treatment of their own people", US Secretary of State Marco Rubio said on Wednesday.
An Iranian diplomatic source told Reuters that any "presence of CENTCOM (US Central Command) or any regional military officials in the talks can jeopardise the process of indirect nuclear talks between Iran and the United States" in Oman.
Iran has said it wants Foreign Minister Abbas Araqchi and US Middle East envoy Steve Witkoff to discuss only the nuclear issue in Oman's capital Muscat.
US President Donald Trump's son-in-law Jared Kushner, who helped mediate in Gaza ceasefire talks, is also due to take part in the discussions.
Tehran's clerical leadership remains deeply worried that Trump may still carry out his threats to strike Iran after a military buildup by the US Navy near Iran.
In June, the US struck Iranian nuclear targets, joining in the final stages of a 12-day Israeli bombing campaign. Tehran has since said its uranium enrichment work has stopped.
The US naval build-up, which Trump has called a massive "armada", has followed a bloody government crackdown on nationwide protests in Iran last month, heightening tensions between Washington and Tehran.
Trump has warned that "bad things" would probably happen if a deal could not be reached, ratcheting up pressure on the Islamic Republic in a stand-off that has led to mutual threats of air strikes.
"While these negotiations are taking place, I would remind the Iranian regime that the president has many options at his disposal, aside from diplomacy, as the commander-in-chief of the most powerful military in the history of the world," White House Press Secretary Karoline Leavitt on Thursday told reporters.
World powers and regional states fear a breakdown in the negotiations would lead to another conflict between the United States and Iran which could spill over to the rest of the oil-rich region.
Iran has vowed a harsh response to any military strike and has cautioned neighbouring Gulf Arab countries hosting US bases in the oil-rich region that they could be in the firing line if they were involved in an attack. Iran has one of the Middle East's biggest stockpiles of ballistic missiles.
Negotiators in Oman will have to navigate Iran's red line on discussing its missile program to reach a deal and avert future military action. Tehran has flatly ruled out talks on its "defence capabilities, including missiles and their range."
In a show of defiance, Iran's state TV said hours before the talks that "one of the country's most advanced long-range ballistic missiles, the Khorramshahr-4," had been deployed at one of the Revolutionary Guards' vast underground "missile cities".
Iran also insists that its right to enrich uranium is not negotiable and demands the lifting of sanctions, reimposed since 2018 when Trump ditched Iran's 2015 nuclear deal with six powers.
The United States, its European allies and Israel accuse Tehran of using its nuclear program as a veil for efforts to try to develop the capability to produce weapons. Iran says its nuclear program is for peaceful purposes only.
