Iran has warned that its campaign to throttle global energy markets could be expanded from the Strait of Hormuz to the vital Red Sea route if US attacks continue, a threat that depends on its Houthi allies in Yemen.

This is why it matters and what it means for the Iran war and the global energy crisis:

How big is the risk to global energy markets?

Closure of the Bab el-Mandeb strait — gateway to the Red Sea — would open a new front in the energy crisis and Iran's overarching conflict with the US.

With Hormuz already disrupted, the Red Sea is now a critical alternative outlet for Persian Gulf oil and other products. Any serious interference at Bab el-Mandeb would raise the prospect that both of the region's major oil export routes could be shut simultaneously.

Iran's partial blockade of Hormuz after Israel and the US attacked it on February 28 disrupted most oil and other exports from the gulf, raising prices and delivering a global energy shock.

Saudi Arabia responded by diverting more than 70 per cent of its normal daily crude exports to the Red Sea port of Yanbu. Shipments from Yanbu averaged four million barrels per day in recent weeks according to data from Kpler and Signal Ocean, up from around 973,000 bpd in the same period last year.

Total volumes of petroleum transiting Bab el-Mandeb amounted to 7.4 million bpd in June, or about seven per cent of global oil output according to Kpler data, up from 4.2 million bpd last year.

That has been a lifeline for the energy market, helping to keep down global oil prices, and Saudi Arabia is considering an expansion of its crude oil pipeline to the Red Sea coast, Reuters reported last week.

Any sustained Houthi disruption to Red Sea shipping including potential attacks on vessels or ports could be a big problem. When the Houthis launched attacks on Red Sea shipping in November 2023, gulf oil exports were flowing freely, so cargoes were not halted. This time, they are being loaded there.

Asef missiles Houthis — Close-up of Asef missiles used by Yemen's Houthi rebels in attacks on shipping in the Red Sea

Who are the Houthis and can Iran make them close Red Sea energy routes?

The Houthis emerged as a military, political and religious movement in north Yemen in the 1990s, fighting guerrilla wars against the government in Sanaa.

They have been in a civil war against the Saudi-backed, internationally recognised, government for more than a decade and attacked gulf neighbours with missiles and drones.

However, a 2022 truce between the country's warring sides has largely held until this week, when Yemen's internationally recognised government said it had struck Sanaa airport to stop an Iranian plane landing.

The Houthis said Saudi Arabia was responsible and fired missiles at Abha airport in the kingdom's mountainous southwest.

A senior Houthi official, politburo member Mohammad al-Farah, then warned in an interview on Iran's Press TV website that if the situation kept escalating, Bab el-Mandeb would be closed.

Iran champions the Houthis as part of its regional "Axis of Resistance", which includes Lebanon's Hezbollah and Iraqi Shi'ite militias, though its ties with the Yemeni movement are less clear than with those other groups.

The Houthis do not recognise Iran's supreme leader as their ultimate religious authority in the same way Hezbollah and the Iraqi groups do. Their motivations are mainly domestic, though the group is ideologically aligned with Iran.

The US says Iran has armed, funded and trained the Houthis with help from Hezbollah. The Houthis deny being an Iranian proxy and say they develop their own weapons. It is not clear how far the group would go purely on Iran's behalf.

The bulker Magic Seas, which was attacked and sunk by Houthi terrorists

What happened when the Houthis attacked Red Sea ships before?

After the October 7, 2023, Hamas attack on Israel, and Israel's devastating campaign in Gaza, the Houthis began firing at Israel and on international shipping in the Red Sea, saying they were doing so in support of Palestinians.

The Houthi attacks in the Red Sea severely disrupted global shipping, prompting Maersk, Hapag-Lloyd and other major companies to divert around Africa - a far longer, more expensive route.

A US-led mission to restore free navigation in the Red Sea involved repeated strikes on Houthi targets and a defensive campaign that shot down hundreds of drones and missiles.

But some Houthi attacks continued until last summer, only ending completely with the Gaza ceasefire in October.

What have they done during the latest Iran war?

While Hezbollah and the Iraqi groups joined the war early with rocket and drone fire after the first US and Israeli strikes on Iran, the Houthis have been comparatively quiet.

The group's leader Abdul Malik al-Houthi said on March 5: "Our fingers are on the trigger at any moment should developments warrant it".

Iranian commanders have repeatedly warned the Houthis could join the war. Revolutionary Guards Quds Force commander Esmaeil Qaani said on June 1 they could choke off the Red Sea.

But the Houthis have mainly been quiet, with a few missile and drone attacks on Israel in late March and early April.

That may have been to hold the threat of a Red Sea closure in reserve for any wider escalation, or because the group was reluctant to enter the war and end its own long ceasefire with Saudi Arabia.

(Compiled by Angus McDowall; editing by Jason Neely, William Maclean)