Yemen’s Houthis said they attacked “sensitive” sites in the Saudi capital with missiles and drones, hours after flames and a large plume of black smoke were seen rising above Riyadh airport on Saturday.

The Saudi government sent air raid alerts to citizens by phone overnight warning of aerial attacks, marking the first time the government has issued such alerts for Riyadh since fighting with the Iran-backed Houthis escalated in July.

Explosions were heard in Riyadh as firefighters battled to extinguish the flames of a burning fuel tank emblazoned with the logo of the Saudi oil company Aramco near the city’s King Khalid international airport.

Residents told Agence France-Presse that hours after the alert, they had heard what sounded like another round of explosions echoing through parts of the Saudi capital.

According to tracking website FlightRadar24, there were “major problems” at the airport and it has been listed under its maximum disruption index.