By Phil Stewart and Idrees Ali

WASHINGTON, Sept 17 (Reuters) - The United States believes Saturday's attack on Saudi Arabia's oil facilities originated in southwestern Iran, a U.S. official told Reuters on Tuesday.

Three officials, speaking to Reuters on condition of anonymity, said the attack involved both cruise missiles and drones, indicating the attack 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 to make the assessments. However, such U.S. intelligence, if it could be shared publicly, could increase pressure on the United States, Saudi Arabia and others to respond.

One of the three officials expressed confidence that Saudi Arabia's collection of materials following the attacks would yield "compelling forensic evidence ... that will point to where this attack came from."

A U.S. team is helping Saudi Arabia evaluate evidence from the attack, which was claimed by Houthi rebels in Yemen who are battling a Saudi-led coalition.

Iran denies any role in the attack on the world's biggest crude oil processing plant, which knocked out half of Saudi Arabia's oil production.

In a sign that U.S. allies remain unconvinced, French Foreign Minister Jean-Yves Le Drian said he was unsure if anyone had any evidence to say whether drones "came from one place or another."

Relations between the United States and Iran have deteriorated since U.S. President Donald Trump pulled out of the Iran nuclear accord last year and reimposed sanctions on its oil exports.

For months, Iranian officials issued veiled threats, saying that if Tehran were blocked from exporting oil, other countries would not be able to do so either.

However, Iran has denied any role in a series of attacks in recent months, including bombings of tankers in the Gulf and strikes claimed by the Houthis.

U.S. Vice President Mike Pence said the United States was reviewing evidence that suggests Iran was behind the attacks on Saudi oil facilities and stands ready to defend its interests and allies in the Middle East.

"We’re evaluating all the evidence. We’re consulting with our allies. And the president will determine the best course of action in the days ahead," Pence said.

At the Pentagon, Defense Secretary Mark Esper singled out Iran as he met Bahrain's crown prince, saying: "As you can see from recent events, Iran continues to violate international norms and instead has chosen to promote instability and danger throughout the region." (Reporting by Phil Stewart and Idrees Ali; Editing by Chizu Nomiyama and Marguerita Choy)