Riyadh vows ‘serious’ response amid Houthi escalation

• Saudi minister says ‘energy security can’t be taken for granted’ • Yemeni rebels warn of ‘swift, painful response’ if US joins war • Iran ‘deeply concerned’, offers to help broker talks • Riyadh airport to remain shut RIYADH: Saudi Arabia vowed a “serious” response after Houthi strikes turned Riyadh’s international airport into a target, killing 18 people in under a week and leaving the world’s biggest oil ex