Saudi Arabia and its coalitions fighting the Iranian-backed Houthi movement in neighboring Yemen announced that they conducted airstrikes targeting rebel missile sites and Iranian military personnel stationed at these sites.

On Monday, the Saudi-led coalition launched several of these airstrikes on Yemen’s capital, which is occupied by the Houthi rebels. This comes just two days after the rebels launched a drone-and-missile attack targeting the Saudi capital of Riyadh. They also launched several others in northwestern Amran province.

The Saudis hit a number of high-value targets including the presidential palace compound, a military school, as well as an airbase close to Sanaa airport. Local citizens reported to the media that loud explosions could be heard across the city.

Saudi officials stated that they conducted more than 12 airstrikes that hit Sanaa on Monday, including six strikes on a military academy north of the capital and four on a military airbase that is within the Sanaa International Airport. 

The Saudi media reported that the head of the Saudi-led coalition, Saudi Colonel Turki al-Maliki, said that “legitimate military targets” of the Houthi movement, were struck in response to the Yemeni militias’ ballistic missile attack on Saturday on Riyadh and other areas of the kingdom.

Maliki added that the targets included sites used by the Houthis for storing and assembling various weapons such as missiles and drones, along with positions manned by Iran’s Quds Force. Both Riyadh and Washington have long accused Tehran of shipping advanced arms to the Houthis, a Zaidi Shiite Muslim group.

Around midnight on Saturday night, Saudi Arabia’s Air Defence Forces intercepted and destroyed a ballistic missile over the capital of Riyadh. Another missile was also intercepted and destroyed over the southern Saudi city of Jizan, which borders Yemen, according to state media.

Saudi Arabia, the United Arab Emirates, and other members of their coalition have waged a five-year campaign to oust the Houthi militia, which is supported by Iran, and return control of the country to the internationally recognized government. 

The Saudis and Iranians have long held a contentious relationship as each strives for regional dominance. Their differences begin with religion: Iran is largely Shia Muslim, while Saudi Arabia sees itself as Islam’s leading Sunni power. After the U.S. invaded Iraq in 2003 and removed Sadam Hussein, a huge Sunni adversary of the Iranians, the Iranians sensed an opening and moved. Iran has been increasing its regional presence ever since.

Since 2011, Iran has benefitted from the civil war in Syria and has put proxy forces in the country to open up a land bridge from Tehran to Lebanon. It has stepped up attacks on shipping in the Gulf — something it continues to deny.

Iran and Saudi Arabia have had no diplomatic relations since an attack on the Saudi embassy in Tehran in January 2016 following the execution, by Saudi Arabia, of Sheikh Nimr al-Nimr, a Shia cleric.

However, the fighting in Yemen has been conducted, until now, strictly by proxies. But the Houthi missile attack on Riyadh has opened another door in the conflict. The Saudis attacked the Quds Forces that are operating with the Houthi rebels and have taken a page out of the Israeli handbook and have made it clear that any attack on Saudi Arabia by the Houthis is the same as an attack by Iran itself. Coincidentally, both Saudi Arabia and Israel were the two biggest critics of the Iran nuclear deal reached by the Obama administration arguing that it didn’t limit Iran sufficiently.

The fighting in Yemen continues despite repeated ceasefire attempts. This has left the country so ridden by disease and malnutrition that United Nations officials have described the war as the world’s worst humanitarian disaster. The most recent escalation in the Middle East’s poorest country has displaced more than 40,000 people since January.  And there are already about 3.6 million who have been forced to flee their homes since the war began five years ago.

Martin Griffiths, the UN Special Envoy for Yemen, said on Sunday that concerns about the escalation of fighting in Marib and rebel attacks on Saudi Arabia have put the civilians caught in the middle at greater risk.

“I am gravely dismayed and disappointed by these actions at a time when the Yemeni public’s demands for peace are unanimous and louder than ever before,” he said.