Yemen’s Iran-aligned Houthis escalated their campaign of strikes against Saudi Arabia on Thursday, trying to shut down the kingdom’s ​airspace and oil facilities with missile fire that has killed three people this week and left a plane badly damaged.

The group said it had struck Riyadh airport and warned workers ‌to stay away from energy plants, which would be targeted.

Shrapnel from an interception or another projectile hit had left the commercial plane belonging to Saudi state carrier Saudia Airlines heavily damaged after catching fire while parked at an airport terminal on Thursday, three people briefed on the matter said.

They said no passengers were believed to have been on the plane at the time but they could not rule out any injuries as a result of the incident. A thick black column of ​smoke and flames was shown rising from Riyadh airport in video verified by Reuters.

Saudi Arabia’s government media office, its civil aviation authority and Saudia Airlines did not respond to emailed requests ​for comment.

Lufthansa and Air India said they were suspending flights to Riyadh after Houthi attacks.

Separately, Saudi officials said debris from an intercepted missile had hit a kindergarten and medical complex in Riyadh.

The Houthis say they are targeting the kingdom over its support for government forces in Yemen’s reignited civil war. Saudi Arabia says it is backing an offensive by Yemen’s UN-recognised government to ​take back territory from the Houthis.

The conflict, now an overspill of the US-Iran war, has added new chaos to a region already aflame, and threatens to draw other states into gruelling mountain fighting, with Syria now considering deploying ​troops to assist Saudi Arabia.

The kingdom has pushed for support from allied nations, including Turkey and Pakistan, even if only in a logistical or defensive capacity, as it tries to reverse Houthi gains in southwest Yemen, by the southern entrance to the Red Sea.

Those requests have taken on more urgency since Saudi authorities said on Wednesday that three people had been killed this week in strikes on Riyadh airport and Abha airport in the southwest.

Delays and Cancellations

The Houthis have made Saudi airports a focus ​of their strikes, and the Saudi-led coalition fighting in Yemen said it had destroyed two ballistic missiles launched by the group towards Riyadh and another towards Khamis Mushayt, home to a major airbase.

According to data ​compiled by aviation analytics company Cirium, almost half of departures from Riyadh airport had been cancelled by 1500 GMT, with 31 more cancellations expected for Friday in all Saudi airports.

Flight tracker FlightRadar24 indicated numerous delayed arrivals and departures at Riyadh’s ‌King Khalid International Airport on Thursday. The airport advised travellers to contact airlines to check the status of flights before trying to travel.

Riyadh is due to host a major meeting of energy policy makers, the WPC Energy Congress, this week.

Several multinational firms based in Riyadh’s King Abdullah Financial District (KAFD) told staff to work from home after loud booms were heard on Thursday morning, five people familiar with the matter said, including some affected staff.

The Saudi government did not respond to a request for comment, and KAFD was not immediately available for comment.

Some diplomats said they had received warnings, and some schools in Riyadh told parents there were orders to shelter in ​place, though authorities did not issue a public alert. ​Reuters could not identify the source of the booms.

Civil War Reignited by conflict in Iran

Yemen’s long-running civil war, which paused with a ceasefire in 2022, has erupted again in recent weeks with the Houthis seizing stretches of the Red Sea coast last month, including along the vital Bab el-Mandeb Strait.

Their threats to shipping and attacks on Saudi cities and infrastructure have ​drawn the kingdom back in. This week, Saudi-backed forces launched a counterattack, taking back some areas around the strait.

However, in a war with several front lines, the Houthis ​have made gains of their own, surrounding Yemen’s third-largest city, Taiz, by taking territory along its main mountain road to Aden.

The fighting, which pits the Houthis against a coalition of Saudi-backed forces under the internationally recognised government, has led to a humanitarian crisis in the poorest Arab state, displacing 200,000 people since the conflict resumed.

One US and one Syrian defence official said Syria was carefully considering options for military support, including defensive aid to protect Saudi Arabia from attacks, or an offensive ⁠deployment to help ​take the fight to the Houthis.

In response to a request for comment to the Syrian foreign ministry, a diplomat said Syria ​stood firmly with Saudi Arabia but that its focus was on domestic priorities such as recovery from its own civil war.

NATO-member Turkey said on Wednesday it was sending mainly defensive and technical support to the kingdom to help in the fight against the Houthis. ​A Turkish official said this was expected to take the form of military equipment, intelligence sharing and weapons systems.