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."

US intelligence services have the capability of determining where the missiles were launched from, the official said, declining, however, to say how many were fired.

"I will not get into that kind of details," the official said.

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.

The president said the United States wanted to help its Saudi ally but he wanted to avoid a war.

Tensions between Iran and the United States and its allies have threatened to boil over since May last year when Trump abandoned a 2015 nuclear deal and began reimposing sanctions in its campaign of "maximum pressure."