A US military blockade of maritime traffic entering and leaving Iranian ports began at 14:00 GMT (15:00 BST) on Monday.
The US says ships coming or going from elsewhere would be allowed to pass through the Strait of Hormuz, a critical waterway Iran effectively closed in response to US-Israeli strikes.
It comes after negotiators from both sides failed to reach a deal to end the war, which began on 28 February but is currently under a two-week truce.
US President Donald Trump has said direct talks with Iran in Pakistan failed because Iran was “unwilling to give up its nuclear ambitions” – but a US official said there had been a much longer list of disagreements, including on Iran’s control of the strait.
Here’s what we know about the US announcement.
After Iran talks falter, the big question is what happens next?
What has Trump said about the blockade?
Trump wrote on social media on Sunday that the US was going to start “BLOCKADING any and all Ships trying to enter, or leave, the Strait of Hormuz”.
“I have also instructed our Navy to seek and interdict every vessel in International Waters that has paid a toll to Iran. No one who pays an illegal toll will have safe passage on the high seas,” he said.
He added that the US will also begin destroying the mines he said Iran has laid in the strait.
“Any Iranian who fires at us, or at peaceful vessels, will be BLOWN TO HELL!” he continued.
Trump said that “at some point” an agreement on free passage will be reached, but “Iran has not allowed that to happen by merely saying, ‘There may be a mine out there somewhere,’ that nobody knows about but them”.
He added in another post that “Iran promised to open the Strait of Hormuz, and they knowingly failed to do so”.
“As they promised, they better begin the process of getting this INTERNATIONAL WATERWAY OPEN AND FAST!” he said.
The US Navy Commander’s handbook on naval operations law from 2022 defines a blockade as a “belligerent operation to prevent vessels and/or aircraft of all states, enemy and neutral, from entering or exiting specified ports, airfields, or coastal areas belonging to, occupied by, or under the control of an enemy state”.
Trump initially said the US Navy would begin the process of blockading the strait “effective immediately”.
The US Central Command (Centcom) later said its forces would begin implementing the blockade at 10:00 EDT (14:00 GMT) on Monday.
A note sent by Centcom to mariners, seen by the BBC, says the blockade will “encompass the entirety of the Iranian coastline to include but not limited to ports and oil terminals,” and applies to all vessel traffic, regardless of flag.
It said neutral vessels are being granted a “grace period” to depart Iranian ports that will expire once the blockade begins, after which time “any vessel entering or departing the blockaded area without authorisation is subject to interception, diversion, and capture”.
Centcom added that it would however permit passage of humanitarian shipments including food, medical supplies and other essential goods, subject to inspection.
Trump said that other countries would be involved in blockading the strait, but did not say which ones. The BBC understands that the UK will not be involved in the blockade.
Trump also told Fox News that Nato had offered to help “clean out” the strait, adding that it would be free to use again “in not too long”.
Trump said the US would bring in minesweepers, and that the UK – a member of Nato – would too.
Prime Minister Sir Keir Starmer has previously said British military mine-hunting systems were already in the region.
A UK government spokesperson said: “We are urgently working with France and other partners to put together a wide coalition to protect freedom of navigation.”
However, Sir Keir said the UK would not join the US blockade.
Three legal experts in the US told the BBC that a blockade could violate maritime law. One also questioned whether a blockade, which is enforced militarily, would violate the current ceasefire agreement.
Why has the US blockaded the strait?
The strait’s geography has allowed Iran to use it as leverage throughout this war, selectively preventing vessels from passing through the narrow waterway and spiking oil prices in the process.
Tehran has been charging huge sums of money for some vessels to pass through.
By closing the strait, Trump could cut off a significant source of revenue for the Iranian government – though this could risk sending oil and gas prices even higher.
He told Fox News that “we’re not going to let Iran make money on selling oil to people that they like and not people that they don’t like”, saying the goal instead was letting “all or nothing” pass through the crucial shipping channel.
Analysts have suggested that the US president’s statement was aimed at building pressure on Iran to make a deal on American terms.
Republican Congressman Mike Turner told CBS that the blockade was a means to force a resolution to the situation.
“The president, by saying we’re not just going to let them decide who gets through, is certainly calling all of our allies and everyone to the table,” he said.
But Senator Mark Warner of Virginia, the ranking Democrat on the Senate Intelligence Committee, told CNN: “I don’t understand how blockading the strait is going to somehow push the Iranians into opening it.”
What could the impact be?
In the near term, Trump’s threat to blockade the strait will only affect a handful of vessels that are still navigating the waterway, shipping expert Lars Jensen told the BBC.
“If this is actually done by the Americans, it will halt a very tiny trickle of vessels. In the greater scheme of things, it doesn’t really change anything,” he said.
Jensen, chief executive of Vespucci Maritime, said Trump’s threat of preventing safe passage for any ships paying tolls to Iran would also have little impact, as any company doing so would already face sanctions for paying the regime.
Most shipping companies will continue to wait and see whether there was a tentative peace agreement and whether that might hold, Jensen said. If that occurred, a slow ramping up of shipping may resume.
Additional reporting by Sareen Habeshian