The Houthi movement that controls northern Yemen vowed on Monday to fire more missiles into Saudi Arabia unless it stops bombing the country, after one of its missiles caused casualties in the Saudi capital for the first time, Reuters reports.
The Saudi-led coalition fighting the Houthis accused them of using Iranian-made missiles. Spokesman Colonel Turki al-Malki said the coalition reserved the right to respond to Iran "at the appropriate time and manner", under international law and within the framework of the United Nations, to protect Saudi Arabia.
The incident threatens to sharply escalate a war that has already unleashed what the United Nations considers the world's most urgent humanitarian crisis. Millions of Yemenis live under threat of mass starvation and disease, at the mercy of combatants who have sometimes cut off food and medical supplies.
Saudi forces said they shot down three missiles over Riyadh shortly before midnight. Debris fell on a home in the capital, killing an Egyptian man and wounding two others.
Air defences also repelled missiles fired at the southern Saudi cities of Najran, Jizan and Khamis Mushait, the coalition said.
The attacks stripped away the sense of calm in Riyadh, a city which until recent months had never quite felt at war.
A Houthi leader hailed the attack, which took place as Yemen marked the third anniversary of the start of the war.
"We praise the successful advance of military capabilities," Houthi political council chief Saleh al-Samad told tens of thousands of supporters in the Yemeni capital Sanaa.
"If they want peace, as we have said to them before, stop your air strikes and we will stop our missiles," he said. "If you continue your air strikes, we have a right to defend ourselves by all means available."
The war pits a coalition of Sunni Arab states friendly to the West against the Houthis, a Shi'ite armed movement sympathetic to Iran.
The Houthis, who deny they are Iranian pawns and say their movement is a national revolution against corruption, control the north of Yemen, including the capital Sanaa.
Saudi Arabia and its allies have been fighting on behalf of an exiled government with a foothold in the south.
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Al-Malki accused Iran of providing the Houthis with ballistic capabilities and said the international community must "work together to combat this dangerous escalation" which threatened security in the region and beyond.
At a press event in Riyadh, he displayed what he said were the remnants of a missile fired into Riyadh alongside an Iranian-manufactured Sayyad 2 missile he said had been part of a seized shipment on its way to Houthi fighters in Yemen.
Independent UN experts reported to the Security Council in January that Houthi missiles they had examined and other military equipment had been manufactured in Iran.
Last year, when the Houthis fired missiles at Riyadh which were intercepted, the Arab coalition responded by shutting Yemen's airports and ports. The United Nations said that blockade raised the prospect of mass starvation before it was partially lifted.
Thousands of air strikes on Yemen, some of which have hit hospitals, schools and markets, killing hundreds of civilians, have brought the coalition little closer to military victory.
Saudi Arabia has said hundreds of its own soldiers and civilians have been killed in Houthi mortar and short-range missile attacks across their rugged southern border.
The United Nations says 10,000 people have died in the war, and three out of four Yemenis — 22 million civilians — need relief aid. More than 1 million cholera cases have been reported, the worst outbreak in modern history.
Western countries have urged Saudi Arabia and its Gulf Arab allies to protect civilians and find a quick end to the war. But they also support Riyadh's argument that it needs to defend itself from cross border strikes and limit the spread of Iranian influence in territory overlooking important trade routes.
The US Pentagon blamed Iran for exacerbating the conflict.
"This follows Iran's pattern of providing advanced weapons to the Houthis," spokeswoman Lieutenant Commander Rebecca Rebarich said in a statement. "Iran has enabled the conflict in Yemen to spill into neighbouring countries and undermines international efforts to resolve the conflict, exacerbating the suffering of the Yemeni people."
French Foreign Minister Jean-Yves Le Drian, appearing to refer to Iran, said in a statement that the transfer of missile capabilities to non-state actors that may be used against states is "irresponsible and contrary to law".
Anwar Gargash, Minister of State for Foreign Affairs of the United Arab Emirates, a Saudi military ally, tweeted: "The message of the Houthi-Iranian missiles is clear. There's no coexisting with a terrorist militia which threatens the stability of our region and is a proxy of Tehran."
Qatar, which has been under months of sanctions from Arab countries including Saudi Arabia and the UAE, also condemned the attacks on Saudi cities as "a violation of international law".
The Saudi military depends on service contracts with Western arms companies to keep its planes flying. Crown Prince Mohammed bin Salman discussed the war in recent visits to US President Donald Trump and British Prime Minister Theresa May.
Diplomats and Yemeni political officials reported this month that the Houthis and Saudi Arabia were conducting secret peace talks after years of UN-mediated dialogue yielded no results.