Saudi Arabia on Sunday raced to restart operations at oil plants hit by drone attacks which slashed its production by half. Iran has dismissed US claims it was behind the assault.
Tehran-backed Houthi rebels in Yemen, where a Saudi-led coalition is bogged down in a five-year war, claimed responsibility for Saturday's strikes on two plants owned by the Saudi state-owned energy company Aramco.
Saturday's explosions set off fires that engulfed the Abqaiq plant, the world's largest oil processing facility, and nearby Khurais, which hosts a massive oil field.
Overall national production has been reduced by half.
Saudi Arabia will use its vast oil reserves to offset disruption to production after the attack, its energy minister said Sunday.
Saturday's drone strikes knocked 5.7 million barrels per day off production, close to six percent of global crude supplies.
The Saudi stock exchange slumped three percent as the week's trading began on Sunday morning.
US blames Tehran for "unprecedented attack"
United States Secretary of State Mike Pompeo pointed the finger squarely at Tehran, saying there was no evidence the "unprecedented attack on the world's energy supply" was launched from Yemen.
"The United States will work with our partners and allies to ensure that energy markets remain well supplied and Iran is held accountable for its aggression," the top US diplomat added.
That drew an angry response from Tehran, where foreign ministry spokesman Abbas Mousavi said: "Such fruitless and blind accusations and remarks are incomprehensible and meaningless."
The remarks were designed to damage Iran's reputation and provide a pretext for "future actions" against the Islamic republic, he said.
Saudi de facto ruler Crown Prince Mohammed bin Salman, whose nation is pitted against Iran in a decades-old struggle for regional dominance, has said the kingdom is "willing and able" to respond to this "terrorist aggression".