SAUDI Arabia’s crown prince has said a ballistic missile launched by Yemen’s Shiite rebels was “direct military aggression by the Iranian regime” as the kingdom ordered the closure of all ports and grounded all humanitarian flights to Yemen.

A Saudi-led military coalition, which has been at war with Yemen’s Houthi rebels for more than two years, earlier tightened an air, land and sea blockade in response to the missile. It was intercepted near Riyadh but marked the rebels’ deepest strike yet into Saudi territory.

State media quoted Crown Prince Mohammed bin Salman as telling UK Foreign Secretary Boris Johnson in a telephone conversation that the strike “may be considered an act of war”.

The state-run Saudi Press Agency carried a statement on the crown prince’s conversation with Boris Johnson.

The kingdom has accused Tehran of supplying the missile fired towards Riyadh’s international airport on Saturday night.

Iran, which supports the Houthis but denies arming them, says it had nothing to do with the attack.

Human Rights Watch described the indiscriminate targeting of a predominantly civilian airport as an “apparent war crime”.

But Sarah Leah Whitson, Middle East director for the New York-based watchdog, added: “This unlawful attack is no justification for Saudi Arabia to exacerbate Yemen’s humanitarian catastrophe by further restricting aid and access to the country.”

Humanitarian flights to Yemen were grounded and ships ordered to leave, resulting in immediate price hikes on the streets of the rebel-held capital, Sanaa.

The move threatens to worsen an already devastating humanitarian crisis in the country, where fighting has killed more than 10,000 civilians and displaced three million. A United Nations official said the organisation’s flights had been cancelled, and it was seeking “to resolve the issue as soon as possible”.

The International Committee of the Red Cross urged the reopening of ports for medical supplies.

The relief agency said a shipment of chlorine tables used to prevent cholera, which has ravaged Yemen over recent months, did not get a clearance at Yemen’s northern border. More supplies are due next week, including 50,000 vials of insulin, the ICRC said.

In announcing the closures earlier this week, Saudi Arabia said it would take into consideration continuing aid efforts. The Saudi-Houthi war dates back to 2014, when the Yemeni rebels and their allies swept down from their northern heartland and seized Sanaa.

This forced the internationally recognised government to relocate to Saudi Arabia.

The Saudi-led coalition controls Yemen’s air space but has made little progress on the ground, where the fighting has been in stalemate for more than a year.

The tiny African nation of Djibouti has become the main transit point for humanitarian flights to Yemen. With the latest measures, however, flights are no longer being given clearance to leave, according to Doctors Without Borders.

The Houthis have vowed to continue targeting Saudi Arabia as well as the United Arab Emirates, a key member of the coalition.

Colonel Aziz Rashed, an army spokesman with a unit allied with the Houthis, warned travellers and travel agencies to stay away from Saudi and Emirati airports as they are considered “legitimate targets”.

He claimed his military experts could develop missiles with ranges exceeding 1500 kilometres (932 miles).