Iran's foreign ministry spokesman said his government had not yet reached a conclusion on the emerging plan, which according to the sources centres on a short-term memorandum rather than a comprehensive peace deal, underscoring deep divisions between the two sides.
Hopes that even a partial deal could lead to the reopening of the Strait of Hormuz have already moved markets, with oil prices sliding again on bets that supply disruptions could ease and global stocks largely holding onto record highs.
Iran and the US have scaled back ambitions for a sweeping settlement as differences persist, particularly over Iran's nuclear program - including the fate of its highly enriched uranium stockpiles and how long Iran would halt nuclear work.
Instead, they are working toward a temporary arrangement set out in a one-page memo aimed at preventing a return to conflict and stabilising shipping through the strait, the sources and officials said.
"Our priority is that they announce a permanent end to war and the rest of the issues could be thrashed out once they get back to direct talks," a senior Pakistani official involved in mediation between the two sides told Reuters.
The proposed framework would unfold in three stages: formally ending the war, resolving the crisis in the Strait of Hormuz and launching a 30-day window for negotiations on a broader agreement, according to the sources and officials.
Iranian Foreign Minister Abbas Araqchi held a phone call on Thursday with Ishaq Dar, his counterpart in Pakistan, which has taken a leading role in mediation efforts.
"We remain optimistic," Pakistan's foreign ministry spokesman Tahir Andrabi told a briefing in Islamabad when asked how quickly a deal could come.
"A simple answer would be that we expect an agreement sooner rather than later."
US President Donald Trump - who has repeatedly played up the prospect of a breakthrough since the war began on February 28 with US-Israeli strikes on Iran - has also struck an optimistic tone.
"They want to make a deal ... it's very possible," he told reporters at the White House on Wednesday, adding later that "it'll be over quickly".
The proposal would formally end the conflict in which full-scale warfare was paused by a ceasefire announced on April 7.
But it leaves unresolved key US demands that Iran suspend its nuclear work and reopen the Strait of Hormuz, the sources said.
In a sign of the US keeping up pressure, it imposed sanctions on Thursday on Iraq's deputy oil minister and three militia leaders over what it said was their support for Iran.
Israel, which has also been fighting Iran-aligned Hezbollah in Lebanon, said on Thursday it had killed a Hezbollah commander in an air strike on Beirut a day earlier, the first Israeli attack on the Lebanese capital since a ceasefire there was agreed last month.
Hezbollah triggered its latest conflict with Israel by opening fire in support of Iran on March 2.
A halt to Israeli strikes in Lebanon is another key Iranian demand in negotiations with the US.
A source briefed on the mediation said the US negotiations were being led by Trump's envoy Steve Witkoff and son-in-law Jared Kushner.
If both sides agreed on the preliminary deal, that would start the clock on 30 days of detailed negotiations to reach a full agreement.
The sources said the memorandum did not mention several demands the US has made in the past which Iran has rejected, such as limits on Iran's missile program and an end to its support for proxy militias in the Middle East including Hezbollah.
The sources also made no mention of Iran's existing stockpile of about 400kg of near-weapons-grade uranium - one of the central concerns of the US.
