The United States warned Saturday that hostilities between Saudi Arabia and Iran-backed Houthis could "escalate rapidly" after a missile attack targeted Riyadh for the first time since the Yemen conflict resumed.

US President Donald Trump cut short a weekend at Camp David, a secluded presidential complex in rural Maryland, to return unexpectedly to the White House on Saturday.

The White House gave no explanation for the early return, which comes as a new threshold was crossed in the conflict between Saudi Arabia and the Houthis, who control a large part of Yemen and are fighting government forces backed by a coalition led by Riyadh.

Loud explosions rang out twice on Saturday in the Saudi capital, where AFP journalists saw a fuel tank bearing the logo of oil giant Aramco on fire near the airport.

"This military conflict has the potential to escalate rapidly," the US State Department warned, adding Ameri...