The move by the hardline IRGC appeared to raise the stakes ahead of the talks that Pakistani officials said will begin on Sunday as both sides seek to advance the interim deal signed by US President Donald Trump and Iranian President Masoud Pezeshkian to end their almost four-month war.
The IRGC warned ships not to approach the waterway, a vital conduit for global oil and gas supplies, citing what it called Israeli "crimes" in Lebanon and a US violation of commitments to establish a ceasefire.
It said vessels' security would be at risk if they approached the strait.
US Central Command said 55 merchant ships transited the strait on Saturday, moving large amounts of cargo and more than 17 million barrels of oil to global markets.
Fox News on Saturday aired an interview with Vance in which he said he was confident the ceasefire agreed in the US 14-point deal with Iran would hold, and that he had seen no evidence that the strait was closed.
"I expect that I will leave sometime in the next couple of days but you know it's always a delicate co-ordination dance and the diplomatic protocols," Vance said.
He added that US negotiators Jared Kushner and Steve Witkoff had been in Switzerland "for a few hours, dealing with some of the technical elements of this negotiation".
"My understanding, talking to Jared and Steve this morning, is things are going well," he added.
Iran's foreign ministry said its negotiators would leave for Switzerland later in the day.
The Iranian delegation was being led by chief negotiator Mohammad Bagher Qalibaf and included Foreign Minister Abbas Araqchi as well as senior security, central bank and oil officials, Iranian media said.
Iranian Foreign Ministry spokesman Esmaeil Baghaei said Iran would press in Switzerland for a fulfilment of commitments, citing past failures by the other side to honour agreements.
One of the conditions for starting 60 days of talks on Iran's nuclear program and other issues is a halt to fighting in Lebanon.
However, Lebanese Civil Defence said that 16 people had been killed by Israeli strikes in Lebanon on Saturday, hours after a truce there took effect.
Israel said it was responding to attacks from Hezbollah, while the Iran-aligned group said it would not allow Israel "freedom of movement" in Lebanon.
Israel, left out of the talks, says it is not party to the Iran-US deal, and will keep its forces in the Lebanese territory it occupies.
A US official had said the truce took effect at 4pm on Friday, and Israeli and Hezbollah sources confirmed the agreement.
Lebanonese news agency NNA said Israeli warplanes and drones ]struck locations across southern Lebanon and the Bekaa Valley on Saturday, both Hezbollah strongholds.
The civil defence service said 16 people were killed in the raids.
An Israeli military official said Hezbollah fired more than 50 projectiles at Israeli forces in southern Lebanon overnight, and that Israel had attacked what it described as Hezbollah targets in response.
A military statement said Israel was committed to the ceasefire and would continue to act against any threat to Israel or its forces.
Hezbollah said it remained committed to the ceasefire but would respond to any attempt by Israel to seize territory or expand its occupation.
Switzerland said it continued to provide a "discreet and reliable setting" at the mountain resort of Burgenstock to facilitate discussions.
Its foreign ministry said no further details would be disclosed about participants and the talks' content.
