The Iran-aligned Houthi group said in an email sent to shipping companies that shipping companies should not load or unload goods at Saudi ports, and that such activity may lead to them being targeted "anywhere."

The email, dated July 20 and sent by the Houthi-run Humanitarian Operations Coordination Center in Sanaa, said: "Ships are prohibited from loading or unloading cargo at or from any Saudi port."

The letter, which was sent to several companies in the sector and seen by Reuters, added: "We strongly recommend that your company exercise due diligence and utmost caution in all its dealings."

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The letter said: "Any such activity will expose violating ships to sanctions. Moreover, they may be subject to targeting anywhere within the operational scope of the Yemeni Armed Forces."

The renewed tension dates back to July 13, when the Houthis announced they had targeted Abha airport with missiles and drones, in response to a strike that targeted Sanaa airport, after which the Yemeni government said it had targeted the runway to prevent an Iranian plane from landing because it had not obtained permission.

This is the most serious escalation between the two sides since a truce was reached in April 2022, which largely held despite its expiry in October of the same year.