The United States has issued satellite images and cited intelligence to back its claim that Iran was behind attacks on major Saudi oil facilities.
Iran denies involvement in Saturday's air attacks, which were claimed by Iran-aligned Houthi rebels in Yemen.
But unnamed US officials speaking to US and international media say the direction and extent of the attacks cast doubt on Houthi involvement.
The incident has cut global oil supplies by 5% and prices have soared.
What is the US saying?
Secretary of State Mike Pompeo blamed Iran at the weekend, without providing any evidence, prompting Tehran to accuse Washington of deceit.
Tweeting on Sunday, President Donald Trump stopped short of directly accusing Iran, but suggested possible military action once the perpetrator was known.
Saudi Arabia oil supply was attacked. There is reason to believe that we know the culprit, are locked and loaded depending on verification, but are waiting to hear from the Kingdom as to who they believe was the cause of this attack, and under what terms we would proceed!
— Donald J. Trump (@realDonaldTrump) September 15, 2019
End of Twitter post by @realDonaldTrump
Unnamed US officials have been speaking to the New York Times, ABC and Reuters.
One official said there were 19 points of impact on the targets and the attacks had come from a west-north-west direction – not Houthi-controlled territory in Yemen, which lies to the south-west of the Saudi oil facilities.
The officials said that could suggest launch sites in the northern Gulf, Iran or Iraq.
A close-up image of damaged tanks at the Abqaiq processing plant appeared to show impact points on the western side.
Officials quoted by the New York Times said a mix of drones and cruise missiles might have been deployed, but that not all hit their targets at Abqaiq and the Khurais oilfield.
ABC quoted a senior US official as saying that Mr Trump was fully aware that Iran was responsible.
What are the oil markets doing?
The oil price has surged.
Brent crude jumped 10% to $66.28 a barrel – the biggest one-day rise since the 1991 Gulf War.
West Texas Intermediate rose 8.9% to $59.75 in Asian trading.
Prices eased slightly after President Trump authorised the release of US reserves.
But there are concerns that higher prices could continue if tensions worsen further.
As for consumers, it is too early to tell if they will see an impact, writes BBC Business reporter Katie Prescott. Any rise could take weeks to feed through to petrol prices.
How has Iran reacted?
Iran has yet to respond to the latest US assertions.
But Iranian Foreign Minister Javad Zarif tweeted on Sunday to deride Mr Pompeo, saying that "having failed at max pressure, Sec Pompeo's turning to max deceit".
He was referring to the Trump administration's stated "maximum pressure campaign" which has targeted Iran with sanctions since Washington pulled out of an international agreement to limit the scope of Iran's nuclear programme.
How did Saturday's attacks unfold?
The attacks targeted Abqaiq, the site of the largest oil processing Read More – Source