Iran sent a new peace proposal to the United States with terms that appeared similar to offers Washington has previously rejected, although a senior Iranian official told Reuters on Monday that the US had softened positions on some issues.
A Pakistani source confirmed that Islamabad, which has conveyed messages between the sides in the war in the Middle East since hosting the only round of peace talks last month, had shared the latest proposal with Washington. But the source suggested progress had been difficult.
The sides "keep changing their goalposts," the Pakistani source said, adding: "We don't have much time."
Iranian Foreign Ministry spokesperson Esmaeil Baghaei confirmed that Tehran's views had been "conveyed to the American side through Pakistan" but gave no details. Washington did not immediately comment.
The Iranian proposal, as described by the senior Iranian source, appeared similar in many respects to Iran's previous offer, which US President Donald Trump rejected last week as "garbage".
It would focus first on securing an end to the war, reopening the Strait of Hormuz - a major oil supply route that Iran has effectively blockaded - and lifting maritime sanctions.
More contentious issues around Iran's nuclear programme and uranium enrichment would be deferred to later rounds of talks, the source said.
However, in an apparent softening of Washington's stance, the senior Iranian source said the United States had agreed to release a quarter of Iran's frozen funds - totalling tens of billions of dollars - held in foreign banks. Iran wants all the assets released.
The Iranian source also said Washington had showed more flexibility in allowing Iran to continue some peaceful nuclear activity under supervision of the International Atomic Energy Agency.
Iran's Tasnim news agency separately quoted an unidentified source as saying the US had accepted waiving oil sanctions on Iran while negotiations were under way.
Iranian officials did not immediately comment on Tasnim's report, which a US official, who declined to be named, said was false.
A fragile ceasefire is in place after six weeks of war that followed US-Israeli airstrikes on Iran. But talks mediated by Pakistan have stalled and Trump has said the ceasefire is "on life support".
Washington has previously demanded Tehran dismantle its nuclear programme and allow shipping through the Strait of Hormuz, which normally carries one-fifth of the world's oil and liquefied natural gas supply.
Iran has been demanding compensation for war damage, an end to a US blockade of Iranian ports and a halt to fighting on all fronts, including in Lebanon, where Israel is battling the Iran-backed Hezbollah militia.
Trump said in a post on Truth Social at the weekend that "the Clock is Ticking" for Iran, adding that "they better get moving, FAST, or there won’t be anything left of them. TIME IS OF THE ESSENCE!"
Trump is expected to meet top national security advisers on Tuesday to discuss options for resuming military action, Axios reported.
Baghaei said Tehran was prepared for all scenarios.
"As for their threats, rest assured that we are fully aware of how to respond appropriately to even the smallest mistake from the opposing side," he told a televised weekly press conference.

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