(Adds U.S. officials)

By Phil Stewart and Idrees Ali

WASHINGTON, Sept 17 (Reuters) - The United States believes Saturday's attack on Saudi Arabia's oil facilities were launched from Iran, two U.S. officials told Reuters on Tuesday, with one of them saying it originated in Iran's southwest.

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

The officials did not provide evidence or detail the U.S. intelligence they used to make the assessments. Such U.S. intelligence, if shared publicly, could add to pressure for a response by the United States, Saudi Arabia and others.

One of the three officials who spoke about the delivery systems for the strike expressed confidence that Saudi Arabia's collection of materials following the strike 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. U.S. officials said they do not believe Houthi claims that the Yemen-based fighters carried out the latest attack.

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

Bruce Riedel, a former CIA analyst and expert on the Middle East, said the United States has an uphill battle convincing allies. Trump himself has at times criticized his own intelligence community, and the United States lost credibility by justifying its 2003 invasion of Iraq on false claims that Baghdad had weapons of mass destruction.

"The burden of proof is on the administration to prove that the Houthis claims are inaccurate; the Houthis clearly had motive," Riedel said.

"Given the history of the administration's trashing its own intelligence community why should we believe them now?"

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 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, adding that Secretary of State Mike Pompeo was traveling to Saudi Arabia.

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; additional reporting by Mark Hosenball Editing by Mary Milliken and Cynthia Osterman)