US-Iran talks in Switzerland planned for Friday were cancelled as fighting flared in Lebanon, creating new uncertainty about the timing of negotiations vital to ensure the reopening of the Strait of Hormuz to global shipping.
US President Donald Trump later told NBC News that he had spoken with Israel and asked it to agree to the ceasefire.
"You just gotta calm down sometimes and use your head," Trump was quoted as saying by an NBC reporter on X, who added that the president had declined to specify whether he spoke with Israeli Prime Minister Benjamin Netanyahu directly.
A senior US official said the ceasefire came into effect around 4 pm Lebanon time following an exchange of fire, adding that negotiators for the US and Qataris had worked out the agreement with help from Iran.
Two sources from Iran-aligned Hezbollah and a senior Israeli official confirmed the ceasefire to Reuters.
"If Hezbollah does not attack us, then for us it is not a time of war," the Israeli official said, adding that Israel would keep its forces in southern Lebanon, where it has occupied an area along Israel's northern border.
Two Lebanese security sources said Israel had carried out a dozen air strikes in the first hour of the ceasefire but none were recorded after 5pm The conflict in Lebanon could weigh on negotiations because ending fighting there is a condition for the broader US-Iran accord.
The memorandum of understanding signed this week by the Iranian and US presidents left discussion of Iran's nuclear program and other tough issues until later, giving the sides 60 days to reach a lasting agreement or extend the interim deal.
Preparations for technical talks to start in the Swiss mountaintop resort of Buergenstock were far advanced when a White House spokesperson said on Thursday that US Vice President JD Vance had dropped plans to attend.
A Swiss foreign ministry statement said the talks had been postponed and that Switzerland remained ready to facilitate the talks and the relevant preparatory work was continuing.
The broad interim deal requires the United States, Iran and their allies to declare an immediate and permanent termination of military operations on all fronts, including in Lebanon.
Israel, left out of the talks, says it is not party to the deal.
Iran's Foreign Minister Abbas Araqchi, in a phone call with his Pakistani counterpart on Friday, said the US would be responsible for any violation of its commitments under the deal, including ending the fighting in Lebanon, his ministry said.
Lebanon was sucked into the regional war when Hezbollah opened fire at Israel on March 2, prompting Israel to launch an offensive against the group and invade the south of the country. Lebanon's health ministry said 18 people had been killed and 33 wounded in heavy air strikes in 11 southern towns since midnight into Friday.
The Israeli military said four soldiers had been killed in an incident in Lebanon, without giving further details.
The memorandum signed this week foresees relief for Iran from economic sanctions, the unfreezing of assets worth tens of billions of dollars and immediate US waivers for its exports of oil. The deal gives negotiators 60 days to agree on the status of Iran's nuclear program, unless an extension is agreed, and to set up a $US300 billion ($A428 billion) reconstruction fund for Iran and other financial incentives.
Trump again defended the deal after criticism in Washington, including from some of his Republican allies in Congress who question whether he conceded too much to end an unpopular war ahead of midterm.
"The War has diminished Iran!" he wrote in social media posts on Friday, adding: "We didn't meet out of desperation, Iran did. They are FINISHED! We'll play out the 60 days. They get no money, not ten cents!"
