The US has started a naval blockade of maritime traffic entering and leaving Iranian ports.

Washington says it will allow ships coming or going from elsewhere to pass through the Strait of Hormuz, a critical waterway off Iran's coast that Tehran effectively closed in response to US-Israeli strikes.

The US will rely on its fleet of naval vessels to control access to Iranian ports, and in turn restrict Tehran's ability to profit from oil exports in an attempt to put pressure on the country.

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 ceasefire.

Trump hopes to put pressure on Iran by targeting two economic drivers - the huge tolls it was demanding ships pay to pass through the Strait of Hormuz, and its oil revenue.

The US president 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.

Iran has responded by calling the blockade "piracy", threatening to retaliate by targeting ports in the Gulf states.

The BBC's security correspondent Frank Gardner said the blockade brings two main risks:

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".

The UN's maritime agency, the International Maritime Organisation, has said no country has a legal right to block shipping in straits used for international transit.

"I do understand there is a conflict going on there, but there is still no legal basis in international law to take any actions to block any strait used for international navigation," Secretary-General Arsenio Dominguez told the BBC.

Donald Rothwell, professor of international law at the Australian National University, said international law would see the US, Israel and Iran as 'belligerents' as opposed to 'neutrals'.

"This means the US, as a belligerent, can legitimately impose a blockade under international law – specifically, the law of naval warfare," he wrote.

The exact locations of US navy ships on deployment are not known publicly. That makes it hard to know exactly where the US will attempt to intercept vessels.

However, the US military is unlikely to position its warships close to Iran's coast due to the risk of missile and drone attacks.

Instead the focus of the blockade is likely to be around the Gulf of Oman, a key transit point for much of the world's oil, including Iranian exports.

US forces will likely use satellite and commercial intelligence to monitor which ships are leaving Iranian ports, and wait for them to sail into the open Indian Ocean before intercepting them.

Satellite imagery from 11 April analysed by BBC Verify shows the USS Abraham Lincoln aircraft carrier positioned at the eastern edge of the Gulf of Oman, roughly 200km (124 miles) to the south of the Iranian coast.

This is the closest to the Gulf that BBC Verify has observed the nuclear-powered warship since the start of the war.

Two other nearby warships visible in the imagery are consistent in size and shape with US Navy guided-missile destroyers.

They are likely to be part of the Lincoln's carrier strike group, but BBC Verify can't identify exactly which vessels are visible.

So far, there have been no confirmed interceptions or seizures of vessels by the US.

US Central Command (Centcom) has said it would permit passage of humanitarian shipments including food, medical supplies and other essential goods, subject to inspection.

Forces have previously used helicopters to rappel onto oil tankers accused of operating illegally in the Caribbean, but the US military has given limited details on how operations will be carried out in Gulf.

Trump has also said that the US will also begin destroying the mines he accuses Iran of having laid in the strait.

The US president added that "at some point" an agreement on free passage will be reached.

Ship tracking data analysed by BBC Verify shows four Iran-linked ships have crossed the Strait of Hormuz despite the naval blockade.

Two of those ships had visited Iranian ports according to MarineTraffic.

The effective closure of the Strait of Hormuz, through which a fifth of the world's energy shipments pass, has sent oil prices surging.

But Iran has continued exporting its own oil - and in doing so, cashing in on the high prices. March was the fifth biggest month for oil exports from Iran over the past year and a half.

The blockade could also have major consequences for Iran's oil customers.

In 2022, around 82% of crude oil and other fossil fuels leaving the Strait of Hormuz were bound for Asian countries, according to the US Energy Information Administration (EIA).

China alone is estimated to buy around 90% of the oil that Iran exports. On Tuesday, its foreign ministry called the US blockade "dangerous" and "irresponsible".

Analysts say the blockade may be designed to force Beijing to become more active in peace talks.

A prolonged blockade could further damage the global economy, pushing up petrol and food prices, as well as limiting access to critical materials like aluminium, helium and feed stocks for fertilisers.

The International Monetary Fund (IMF) warned on Tuesday that the global economy was at risk of recession if the war continued and high energy prices persisted.

The Fund also said the war's energy shock would hit the UK the hardest of the world's advanced economies.

Civilians in Iran could also be impacted, with the country reliant on grain and wheat imports, creating potential food security issues.

The strait's geography has allowed Iran to use it as leverage throughout this war, using threats to selectively prevent vessels from passing through the narrow waterway.

About 3,000 ships a month usually sail through the strait each month but this dramatically decreased to just a handful a day during recent hostilities.

The strait is also a vital channel for imports to the Middle East, including food, medicines and technological supplies.

The disruption has created a fuel crisis in Asia, a region heavily reliant on Middle Eastern oil.

Governments have ordered employees to work from home, cut the working week, declared national holidays and closed universities early in order to conserve their supplies.

Prime Minister Sir Keir Starmer has said UK forces will not be involved in enforcing the blockade.

UK minesweepers and anti-drone capabilities will continue operating in the region but British naval ships and soldiers will not be used to block Iranian ports.

Trump has said that other countries would be involved in blockading the strait, but did not say which ones.

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".

Additional reporting by Barbara Metzler, Frank Gardner, Dearbail Jordan, Joshua Cheetham and Shruti Menon

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China, the biggest buyer of Iranian oil, says the US naval blockade undermines an "already fragile ceasefire".

Four vessels with links to Iran have crossed the Strait of Hormuz despite the US naval blockade.