Earlier on Tuesday, Iran’s Fars news agency cited a military source as saying the Strait of Hormuz remained closed. It said no vessels would be permitted to pass while what the source described as hostile US actions continued.

Last week the number of tankers transiting the strait fell to two-month lows as hostilities escalated. Vessel crossings via the strait dropped further at the start of this week, shipping data showed.

A total of four commodity vessels crossed the strait on Monday, mostly on the Iranian route, down from seven the previous day, Kpler data showed. The ​four vessels included two tankers that exited the ⁠strait as well as two vessels that entered.

There were no visible very large crude carrier (VLCC) or liquefied natural gas tankers passing through the strait on Monday.

Shipping industry sources have said vessels are increasingly switching off their public AIS tracking transponders, making it difficult to determine the full number of ships crossing the waterway.

Before the war, around 130 vessels crossed the Strait of Hormuz each day. We can now bring you details of an email sent to companies by Yemen’s Iran-aligned Houthis regarding their Saudi blockade.

It said shipping conzmpanies should not load or discharge cargo at Saudi Arabian ports and such activity may result in being targeted by Houthi militia.

“Vessels are banned from loading or discharging cargo at or from any Saudi ports,” said the July 20 email, which was received by multiple shipping companies.

“We strongly recommend that your company exercise due diligence and the utmost care in all its dealings,” the email, which was sent by the Houthis’ Sanaa-based Humanitarian Operations Coordination Centre.

“Any such activity would expose the violating vessels to sanctions. Furthermore, they may be subject to targeting in any location,” within the Houthi’s operational reach, according to the email.

Bab el-Mandeb is a narrow waterway linking the Red Sea to the Gulf of Aden through which Saudi oil exports and a substantial share of global shipping pass.

Any serious interference at Bab el-Mandeb, coming after months of disruption in the Strait of Hormuz, would raise the prospect that both of the region’s major oil export routes could be shut simultaneously.

A full closure of the Bab el-Mandeb strait would halt Saudi oil exports to Asia and could reduce global oil supply by 7 per cent.

The Houthis have already shown they can choke global commerce through Bab el-Mandeb.

After the Gaza war erupted in October 2023, the Iran-backed group launched attacks on commercial shipping in the Red Sea, saying it was targeting vessels linked to Israel, in solidarity with Palestinians.

The campaign forced major shipping companies to reroute vessels around southern Africa, raising transport costs, and prompting US and British airstrikes as well as a multinational naval mission to protect shipping.

On Monday, Yemen’s Iran-aligned Houthis said they would impose a naval blockade on Saudi Arabia. In their statement, the Houthis’ armed forces said the blockade was “based on the equation of ‘an eye for an eye’, effective immediately” in response to what they called “an unjust and oppressive siege” imposed on Yemen by the Saudis.

The Saudi foreign ministry condemned the allegations and said Saudi would take “all necessary measures” to protect its ships.

The Saudi-led coalition in Yemen vowed to respond to the threat to its commercial ships with firmness. It also said it began implementing protection measures for its ships moving through the Bab el-Mandeb Strait, a key export route for Saudi oil after the closure of the Strait of Hormuz.

Iran had been pressing the Houthis to close the Bab el-Mandeb gateway to the Red Sea if the US continued to attack Iranian power infrastructure. US forces bombed targets in southern Iran overnight after President Donald Trump said Tehran would pay dearly for the deaths of American soldiers.

Iran responded with renewed attacks across the Gulf, targeting U.S. sites in Bahrain, Kuwait and Jordan.

The exchanges came a day after Yemen’s Iran-aligned Houthis said they would impose a naval blockade on Saudi Arabia, opening a potential new front in the war and raising the threat to global energy supplies and trade beyond the Gulf.

The United Kingdom Maritime Trade Operations agency said a tanker in the Strait of Hormuz reported being struck by an unknown projectile, forcing its crew to abandon the vessel and board a lifeboat.

Meanwhile, Iran’s Revolutionary Guards said two oil tankers had caught fire after explosions while attempting to transit the southern route of the strait. It was not immediately clear when those incidents took place.

It is a busy day in Washington, where Lebanese President Joseph Aoun will meet Trump at the White House later to present a plan on how to disarm Iran-backed militant group Hezbollah and secure Israel’s withdrawal from Lebanon.

U.S. Secretary of Defence Pete Hegseth is also due to appear later in front of the Senate Appropriations Committee to answer questions about a budget request from Trump that includes $60 billion to fund the war on Iran.

• A Reuters report