US Secretary of State Mike Pompeo has blamed Iran for Saturday’s drone attacks on Saudi oil facilities.
He dismissed a claim by Yemen’s Iran-backed Houthi rebels that they had attacked the two facilities, run by state-owned company Aramco.
Iran said the accusations were “blind, incomprehensible and meaningless”.
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.
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, in opposition to the Houthi rebel movement.
The attacks also come against a backdrop of continuing tension between the US and Iran, following US President Donald Trump’s abandonment of a deal limiting Iran’s nuclear activities and reinstatement of sanctions.