The official, who declined to be identified, said the United States was gathering evidence about the attack to present to the international community, notably European allies, at the UN General Assembly next week.

Asked if Washington was certain that the missiles had been launched from Iranian soil, the official answered: "Yes."

The weekend strikes on Abqaiq -- the world's largest oil processing facility -- and the Khurais oil field in eastern Saudi Arabia have roiled global energy markets.

Yemen's Iran-aligned Huthi rebels claimed responsibility for Saturday's attacks but Saudi Arabia accused Iran and President Donald Trump also singled out Tehran.

"Certainly, it would look to most like it was Iran," Trump said Monday.