Iran's top joint military command accused the US of violating the April 7 ceasefire by targeting an Iranian oil tanker and another ship entering the Strait of Hormuz chokepoint on Thursday.
The US military said it acted in self-defence after Iran launched "unprovoked" attacks on US Navy destroyers that were passing through the strait. It said Iran did not hit any US assets in the attack.
Air defences were activated in Tehran, the semiofficial news agency Mehr reported.
The renewed hostilities broke out as Washington was awaiting Iran's response to a US proposal that would stop the fighting but leave the most contentious issues, such as Iran's nuclear program, unresolved for now. Iran's foreign ministry spokesperson said Tehran had not yet reached a conclusion on the emerging plan.
The proposal does not address key US demands that Iran suspend its nuclear work and reopen the Strait, which before the war handled one-fifth of the world's oil and gas supply.
The proposed framework would unfold in three stages: formally ending the war, resolving the Strait of Hormuz crisis and launching a 30-day window for negotiations on a broader agreement, according to the sources and officials.
Separately, the US 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-backed Hezbollah in Lebanon, on Thursday said it had killed a Hezbollah commander in an airstrike on Beirut a day earlier, the first Israeli attack on the Lebanese capital since a ceasefire there was agreed to in April.
A halt to Israeli strikes in Lebanon is a key Iranian demand in negotiations with Washington.
Hopes that even a partial peace 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, sources and officials said.
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.
12°C