The US and Iran are set to hold peace talks in Pakistan, which has been mediating, but remain deeply divided on key issues, even though President Donald Trump has said proposals presented by Tehran were a "basis" for talks.
Each side is sticking to competing demands for a deal to end the war that could shape the Middle East for generations.
An Iranian delegation is due to arrive in Islamabad for talks based on a 10-point proposal, which shows little overlap with a 15-point plan Washington previously put forward, suggesting there will be major gaps to bridge.
For example, Iran's proposal includes a demand to enrich uranium, which Washington previously ruled out and Trump has insisted is non-negotiable. The 10 points also do not address Iran's missile capabilities, which Israel and the US have both said must be dramatically curtailed. Tehran has said its formidable missile arsenal is non-negotiable.
One Pakistani official in the region said Iran could expect to secure many of its demands, with a focus on reconstruction, reparations and sanctions relief, but could not expect to secure an agreement on uranium enrichment.
Previous talks have focused on Iran's nuclear programme and missiles. Those issues have now been overshadowed by the fate of the Strait of Hormuz, a chokepoint through which a fifth of the world's oil and liquefied natural gas flows.
Iran's effective closure of the waterway since the beginning of the war on February 28 has rattled the global economy, pushing up oil prices.
Tehran has indicated that under a permanent peace deal, it would seek to charge a fee for ships transiting the strait.
Trump had threatened to devastate Iran if Tehran did not agree to a ceasefire deal and to reopen the strait.
There was no sign Iran had lifted its blockade of the waterway, which has caused the worst disruption to global energy supplies in history, and Tehran said there would be no deal as long as Israel was striking Lebanon.
Iran's Supreme National Security Council said in a statement that Washington had agreed to accept Iran's 10-point plan and that "the United States has, in principle, committed to the plan.
As the two sides prepare to hold negotiations, Trump vowed to retain military assets in the Middle East until a peace deal with Iran is reached and warned of a major escalation in fighting if it failed to comply.
Although Trump has declared victory, Washington did not achieve the aims he had announced to justify the war at its outset.
Iran is unlikely to make major concessions on those points, and it has indicated it can patiently keep on fighting, with the Strait of Hormuz giving it economic leverage over an enemy with superior firepower. Israel, which has been pounding Hezbollah in Lebanon in a parallel conflict, sees Tehran as an existential threat.
The question of whether the ceasefire covers Israel's war against Hezbollah has become a sticking point that threatens the truce.
The US and Israel say Lebanon is not included in the agreement, while Iranian President Masoud Pezeshkian has said a halt in hostilities in Lebanon was an essential condition of Tehran's deal with Washington.
Israel said it had agreed to a ceasefire with Iran, but said the deal did not include halting military action in Lebanon.
Michael Georgy
The writer is a Co-Middle East Breaking News Editor and Lead Writer for Thomson Reuters
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