Thursday’s attacks on Riyadh airport killed three people, Saudi authorities said on Friday, as Yemen’s Houthi group stepped up missile strikes on the kingdom and tried to shut down its airspace in an escalating war triggered by the US-Iran conflict.

The strikes, which also injured several people, damaged a Saudia Airlines plane and sent smoke billowing over the airport, led many airlines to cancel flights to the Saudi capital days before it is due to host a major meeting of global oil chiefs.

Saudi airstrikes hit Houthi targets in the Yemeni capital Sanaa overnight, Houthi officials and affiliated media said, part of an intensifying air war against the Iran-allied group aimed at reversing its recent territorial gains, threats to international shipping, and attacks on the kingdom.

A Saudi-led coalition backing government forces in Yemen said it had destroyed three missile launch platforms used by the Houthis and warned it would respond firmly to attacks on civilians.

The Houthis, who have encircled Yemeni government forces in Yemen’s third-largest city of Taiz and are trying to defend positions on the Red Sea coast against a Saudi-backed offensive, had already killed three people with strikes on airports in the kingdom earlier this week.

The Saudi civil aviation authority said air traffic at Riyadh airport had returned to normal and flight tracking site flightradar24 showed a gradual resumption of flights, including from some Middle Eastern airlines.

Saudia Airlines said one of those killed on Thursday was an employee of theirs.

Bitter fighting inside Yemen

Yemen’s civil war has rapidly emerged as the fiercest front in a wider regional conflagration triggered by the US and Israeli attack on Iran that started on February 28.

The conflict is critical for global energy markets, with the Houthis firing on shipping in the Red Sea and its southern gateway the Bab el-Mandeb Strait, further squeezing Gulf oil supplies with Iran partly blocking the Strait of Hormuz.

Last month the Houthis staged a lightning advance down Yemen’s Red Sea coast to take full control over the coastline around the strait, prompting Saudi Arabia to re-enter the long-dormant conflict and back a government counteroffensive.

On Monday government forces took back territory around the Bab el-Mandeb coastline after intense Saudi airstrikes and fighting continues there.

Saudi Arabia’s ambassador to Yemen, who has managed the kingdom’s engagement in Yemen for over a decade, said in a social media post on Friday that Bab el-Mandeb had been cleared of Houthis.

Reuters was not able to verify that and Houthi forces remain in Mocha, a little way up the coast, with government forces now focused on a battle in nearby hills that overlook the region.

The Houthis have also managed to cut off a key road connecting Taiz, a city of more than half a million people with a garrison of thousands of pro-government fighters, with advances in mountain districts.

The group has announced repeated rounds of ballistic missile fire at Saudi Arabia, proclaiming it would shut down Saudi airspace and warning workers at oil facilities that it would strike those too.

Saudi Arabia is seeking assistance from Pakistan and Turkey, its partners in a mutual defence pact, to help ward off Houthi attacks, and has pushed for wider help from allies to aid it in reversing the group’s gains inside Yemen.

Threats to Saudi airspace

The Houthis on Friday repeated their threats against aircraft flying over Saudi Arabia, except above the holy Muslim cities of Mecca and Medina, describing Saudi airspace as a theatre of operations for its missile forces.

While some regional airlines were still operating flights through Riyadh, many others were still suspended and flightradar24 showed many cancellations and delays.

The European Union Aviation Safety Agency said it was extending the area in which it advised airlines not to fly, citing the recent attacks on Riyadh airport and risks to civil aviation. An aviation risk monitoring advisory body, OPSGROUP, advised against flying over the kingdom.

The US embassy in the United Arab Emirates meanwhile warned Americans to exercise increased vigilance, citing Houthi attacks on civilian targets in Saudi Arabia and saying the hostilities could rapidly escalate.

Riyadh is due to host the WPC Energy Congress, a major meeting of oil ministers, other government officials and oil company executives, from Sunday and it was still advertising the event on Friday.

Italian energy minister Gilberto Pichetto Fratin said he would not attend in person, but take part via video, without mentioning the reason for his decision.