Pakistan, Sept. 20 -- A large plume of smoke was visible near Riyadh's main airport on Saturday after booms were heard and Saudi Arabia sent alerts overnight warning of potential danger around the capital, amid an escalation in attacks by Yemen's Houthi rebels.

This is the first time since Yemen's long-standing civil war resumed that it has threatened the Saudi capital, further shaking a global economy already under pressure by nearly seven months of war between the United States and Iran.

Reuters video and photographs showed a huge pillar of black smoke rising in the area, with the airport highway, main terminal building and other facilities in the foreground. At one point in the video, flames were visible rising from behind what appea...