US Secretary of State Mike Pompeo has blamed Iran for Saturday's drone attacks on Saudi oil facilities.He dismissed a claim by Yemeni Houthi rebels that they had attacked the two facilities, run by state-owned company Aramco.
Saudi Arabia's energy minister said the strikes had reduced crude oil production by 5.7 million barrels a day - about half the kingdom's output, reports BBC.
Correspondents say they could have a significant impact on world oil prices.
TV footage showed a huge blaze at Abqaiq, site of Aramco's largest oil processing plant, while a second drone attack started fires in the Khurais oilfield.
The Saudis lead a Western-backed military coalition supporting Yemen's government, while Iran backs the Houthi rebels.
If the rebels were responsible for the attacks, their drones would have had to fly hundreds of miles from Yemen into central Saudi Arabia.Meanwhile experts are investigating whether the attacks could have been carried out from the north - either by Iran or its Shia allies in Iraq - using cruise missiles rather than drones, the Wall Street Journal reports.
Tensions between the US and Iran have escalated in recent months since US President Donald Trump abandoned a deal limiting Iran's nuclear activities last year and reinstated sanctions.