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.
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