Pentagon Correspondent Carla Babb and Katherine Ahn of VOA's Persian Service contributed to this report.
There is growing evidence that a drone and missile attack launched against Saudi Arabian oil fields under the cover of darkness originated in Iran.
Houthi rebels in Yemen, backed by Iran, originally claimed responsibility for Saturday's attack, knocking out half of the country's oil production. But U.S. officials say the available evidence is showing that is not possible.
"Our working assumption is that this did not come from Yemen or Iraq," a U.S. defense official told VOA Tuesday, adding that a U.S. forensic team is on the ground working with the Saudis to examine the remnants of the missiles.
"We think that evidence will be compelling," the official added.
Separately, other U.S. officials say the evidence that already has been collected is conclusive and points directly to Iran.
Unnamed officials told NBC News on Tuesday that more than 20 drones and missiles used in the attack on the Saudi oil facilities were launched from Iran.
Other officials told CBS News that at least one of the missiles flew through Kuwait's air space as it headed south toward Saudi Arabia, while Reuters quoted officials as saying the attacks originated in southwestern Iran.
Officials also said investigators are examining a missile-guidance mechanism found in Saudi Arabia and a mostly intact cruise missile that apparently failed to reach its intended target.
U.S. Chairman of the Joint Chiefs of Staff Joseph Dunford, speaking on the record to reporters in London, said Saturday's attack looked different from those previously carried out by the Yemen's Houthi rebels.
He said the U.S. does not have any overhead imagery of the attacks, but that in addition to the forensics team, Washington is providing additional support to Riyadh and will let the Saudis make their own assessment.
Despite the conclusions drawn by a growing number of U.S. officials, Iran on Tuesday continued to deny any role in the attack.
"US is in denial if it thinks that Yemeni victims of 4.5 yrs of the worst war crimes wouldn't do all to strike back," Iranian Foreign Minister Javad Zarif wrote Tuesday on Twitter.
The conclusions reached by U.S. intelligence and military officials came as Secretary of State Mike Pompeo headed to Riyadh for discussions with Saudi Arabian officials.
Vice President Mike Pence said in a Washington speech that the top U.S. diplomat would be discussing "our response" to the attack, with both Saudi Arabia and U.S. President Donald Trump already having signaled they believe Iran was responsible even before the definitive conclusions were reached.
Meanwhile, Saudi's energy minister, Prince Abdulaziz bin Salman, announced Tuesday that half of the production cut by the attacks has been restored and that the kingdom expects to be producing 11 million barrels a day by the end of September, compared to 9.6 million before the attacks.
Earlier, Saudi King Salman said his country is capable of defending itself against the "cowardly attacks" but called on the international community to "clearly confront" the perpetrators.
Other Saudi officials also reiterated Riyadh's accusation that Iranian weapons were used in the attack, but offered no evidence of its claim.
Trump has offered mixed signals about a possible U.S. response to the attack on Saudi Arabia, a U.S. ally.
"I'm not looking to get into new conflict, but sometimes you have to," he said. "That was a very large attack, and it could be met by an attack many, many times larger."
"Certainly, it would look to most like it was Iran," he concluded.
With Trump blaming Iran for the attacks, Iran's supreme leader, Ayatollah Ali Khamenei, on Tuesday dismissed the possibility of negotiating with Washington over its nuclear program unless the U.S. returned to the 2015 international pact that restrained Iran's nuclear weapons program in exchange for lifting economic sanctions against Iran.
In withdrawing the U.S. from the accord last year, Trump reimposed the sanctions, hobbling the Iranian economy. There had been suggestions in recent days that Trump could meet next week with Iranian President Hassan Rouhani when both are at the annual United Nations General Assembly.
"Iranian officials, at any level, will never talk to American officials," Khamenei said, adding that Trump's attempt to link Iran to the drone attacks was "part of their policy to put pressure on Iran."
With the initial uncertainty about responsibility for the drone attacks, U.S. House Speaker Nancy Pelosi asked the Trump administration to brief all 435 members of the House of Representatives on its intelligence findings.
The attacks spiked world crude prices Monday by about 15%, but prices retreated by as much as 6% on Tuesday on reports that Saudi Aramco, the Saudi oil company whose facilities were attacked, could return its production to normal more quickly than first thought.
Ashley Peterson, an oil market analyst at Texas-based energy industry consulting company Stratas Advisors, told VOA Persian one reason oil prices have not risen further is that the latest reports on global crude stockpiles show the markets have been well supplied.
"Now there is an opportunity for [oil producers] Russia, the United Arab Emirates and the U.S. to step in and fill a little bit of this gap [resulting from the Saudi attacks]," she said. "Saudi Aramco, China, Japan and South Korea also have said they have stockpiles ready to go."
Peterson said in the short term, the oil market is likely to be focused on uncertainty about geopolitical reactions to the attacks in Saudi Arabia.
"Geopolitical uncertainty is certainly nothing new in the Middle East," said Michael Hewson, chief market analyst at CMC Markets UK, in a comment cited by the French news agency.