The Iranian military says it has closed the Strait of Hormuz again over Israel's attacks on southern Lebanon - although the US military has disputed the claim.
Iran said deadly Israeli strikes in Lebanon were a breach of Tehran's agreement with the US to end the war.
The US-Iran deal includes the reopening of the Strait of Hormuz - a shipping channel through which about 20% of the world's oil and liquefied natural gas passes.
After Iran's statement, a US Central Command spokesperson, Tim Hawkins, told media outlets that "traffic continues to flow", with US forces "monitoring the situation to ensure this remains the case" and that "Iran does not control the Strait of Hormuz".
Later on Saturday, US Vice-President JD Vance departed Washington for direct US-Iran talks in Switzerland on Sunday.
He told reporters he hoped to make progress "on the nuclear issue" and on the "Lebanon ceasefire issue".
Asked about clashes between Israel and Hezbollah and Israeli air strikes in southern Lebanon, Vance said: "Things are actually getting better there, and things are slowing down a little bit."
"It's going to be something we're just going to have to continuously manage to ensure that Israel and Lebanon are both safe and secure. That's fundamentally the goal of this, to make the whole region safe and secure," he said.
Iran's foreign ministry spokesman Esmail Baghaei said his country would be "demanding that the other side fulfil its commitments".
Pakistan's Prime Minister Shehbaz Sharif will attend the start of the talks, his office told the BBC. Pakistan has acted as a mediator throughout the war, and hosted a previous round of negotiations between the US and Iran in its capital, Islamabad, in April.
Earlier this week the US and Iranian presidents signed an initial agreement aiming to end the war, including in Lebanon, with immediate effect. It includes a commitment to further talks to reach a final deal over the next 60 days.
On Saturday Donald Trump posted on social media that the US could impose its own tolls on shipping in the Strait of Hormuz if the US and Iran did not reach a negotiated deal.
Justifying its announcement that it was closing the strait, the Iranian military accused the US of violating the US-Iran deal by not implementing the first clause of their 14-point memorandum of understanding, which agrees to "the immediate and permanent termination of military operations on all fronts, including in Lebanon".
Tracking data monitored by BBC Verify suggested that at least five tankers passed through the Strait on Saturday while several vessels appeared to have made U-turns in the area. However earlier on Saturday Centcom said commercial ship traffic through the strait had increased on Saturday, with 55 merchant ships transiting.
The Iranian announcement came after at least 20 people were reportedly killed by Israeli airstrikes on southern Lebanon, less than 24 hours after a new ceasefire between Israel and Hezbollah was announced.
Lebanon's health ministry said 4,057 people had been killed since the re-start of conflict between Israel and Hezbollah on 2 March.
Israel and Hezbollah have since accused each other of repeatedly violating Friday's ceasefire.
On Saturday the Israeli military said it had struck "dozens" of targets from Hezbollah, after it said the Iran-backed group fired more than 50 projectiles at Israeli forces in the region.
An Israeli strike killed a family of four in the town of Barich, Lebanese state media reported.
Israel's military also said an Israeli soldier was killed in battle in southern Lebanon on Saturday.
Israeli officials have previously said that they had no intention of withdrawing their forces from Lebanon and insisted the conflict with the Iran-backed group Hezbollah was separate from the war on Iran.
Hezbollah meanwhile said Israeli attacks in Lebanon were an attempt to "sabotage" the broader US-Iran deal.
The US government has criticised Israel's ongoing operations in Lebanon, which was drawn into the war when Hezbollah launched rockets into Israel in retaliation for US-Israeli strikes that killed Iran's supreme leader.
Iran effectively blocked the Strait of Hormuz after the US and Israel attacked the country on 28 February - sending shockwaves through global energy markets.
The strait is deep enough for the world's biggest crude oil tankers, and is used by major Middle Eastern oil and liquefied natural gas producers, as well as their customers.
In 2025, about 20 million barrels of oil and oil products passed through the strait per day, according to estimates from the US Energy Information Administration. That is nearly $600bn (£447bn) worth of energy trade per year.
