The United States believes the attacks that crippled Saudi Arabian oil facilities last weekend originated in southwestern Iran, a U.S. official told Reuters on Tuesday, an assessment that further increases tension in the Middle East.

Three officials, speaking to Reuters on condition of anonymity, said the attacks involved both cruise missiles and drones, indicating that they involved a higher degree of complexity and sophistication than initially thought.

The officials did not provide evidence or explain what U.S. intelligence they were using for the evaluations. Such intelligence, if shared publicly, could further pressure Washington, Riyadh and others to respond, perhaps even militarily.

Iran denies involvement in the strikes. Iran’s allies in Yemen’s civil war, the Houthi movement, claimed responsibility for the attacks. The Houthis say they struck the plants with drones, some of which were powered by jet engines.