After years of war that has seen a Saudi-led coalition battling Iran-backed Huthi rebels impose a blockade on Yemen, the UN has already warned that seven million people in the country are on the verge of starvation. But now a decision by Riyadh and its allies to tighten the screws on all land, sea and air borders in response to a Huthi missile attack has sent prices spiralling further -- and ratcheted up fears of a looming famine.