The official briefed on the negotiations, who is not from Iran, said a new draft peace proposal for the eight-month-old conflict was presented on Sunday and was being considered by Tehran, with a response expected by the end of the week.
The proposal presented by Qatar sets a seven-day timeline addressing both Tehran's and Washington's concerns, laying out daily confidence-building steps that each side must take, the official briefed on the negotiations said.
The timeline could begin early next week and several countries would act as guarantors of the deal, the official added.
Shortly afterwards, a senior Iranian official involved in the talks said Qatar had presented only "unacceptable ideas and not an offer", and that Tehran's policy on showing no flexibility over the nuclear issue remained unchanged.
Washington wants Iran's decades-long nuclear program to be part of negotiations to end the war from the outset, while Iran says its conditions - which include an end to the US blockade of Iranian ports and the lifting of sanctions on Iranian oil exports - must be met first.
In late September, Iran presented another seven-day proposal under which Foreign Minister Abbas Araghchi said the Strait of Hormuz could be reopened if certain conditions were met. US President Donald Trump rejected it, prompting further Qatari mediation.
The draft discussed in Tehran on Sunday was different from that proposal, the official briefed on the negotiations said, though the details of the latest plan were not immediately clear.
Araghchi said Iran continued to give diplomacy a chance to succeed and raised the possibility of a seven-day plan proposed by Tehran being completed in less than a week if implemented.
He added Tehran remained in close contact with intermediaries.
Earlier mediation efforts to end the war that the US and Israel launched on February 28 helped produce an interim peace agreement in June, but it quickly unravelled.
In recent weeks, Yemen's Iran-aligned Houthi forces have launched repeated attacks against Saudi Arabia - a key US ally and the world's largest crude exporter - opening a new front in the eight-month-old war.
Oil prices rose more than one per cent to more than $US115 per barrel on Wednesday after a Houthi attack on Saudi Arabia's largest oil refinery, Ras Tanura, stoking concerns over supply risks.
Fighting with the Houthis intensified last month as the militant group advanced against Saudi-backed forces towards the Bab el-Mandeb Strait, another important shipping lane on the other side of the Arabian Peninsula from Hormuz.
The official briefed on the negotiations said there were concerns ahead of Sunday's meeting in Tehran that Israeli Prime Minister Benjamin Netanyahu, fighting for another term in a general election, could resume attacks on the Islamic Republic this week.
On Friday, US President Donald Trump repeated threats to strike Iran again, while Iranian security officials told Reuters Tehran had drawn up detailed plans to attack Gulf undersea cables if the US and Israel resumed attacks.
