Drone attacks claimed by Yemen’s Houthi rebels struck two key oil installations inside Saudi Arabia on Saturday, damaging facilities that process the vast majority of the country’s crude output and raising the risk of a disruption in world oil supplies.

The attacks immediately escalated tensions in the Persian Gulf amid a standoff between the United States and Iran, even as key questions remained unanswered — where the drones were launched from, and how the Houthis managed to hit facilities deep in Saudi territory, some 500 miles from Yemeni soil.

Secretary of State Mike Pompeo accused Iran of being behind what he called “an unprecedented attack on the world’s energy supply” and asserted that there was “no evidence the attacks came from Yemen.” He did not, however, specify an alternative launch site, and the Saudis themselves refrained from pointing the finger directly at Iran.

President Trump condemned the attack in a phone call with Saudi Crown Prince Mohammed bin Salman and offered support for “Saudi Arabia’s self defense,” the White House said in a statement, adding that the United States “remains committed to ensuring global oil markets are stable and well supplied.”