US Secretary of State Mike Pompeo condemned Iran following drone attacks on oil facilities in Saudi Arabia, which had been blamed on Yemeni rebels.
Yemen's Iran-aligned Houthi group attacked two plants at the heart of Saudi Arabia's oil industry, including the world's biggest petroleum processing facility, in a strike that three sources said had disrupted output and exports.
The top US diplomat did not specifically name Tehran as the perpetrator of the attacks that led to fires at two key Saudi Aramco facilities, but said, "Iran has now launched an unprecedented attack on the world's energy supply."
"There is no evidence the attacks came from Yemen," Mr Pompeo said on Twitter.
"We call on all nations to publicly and unequivocally condemn Iran's attacks. 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."
Two sources close to the matter said five million barrels per day of crude production had been impacted - close to half of the kingdom's output, or 5% of global oil supply - but did not elaborate.
The pre-dawn drone attack on the Saudi Aramco facilities sparked several fires, although the kingdom, the world's largest oil exporter, later said these were brought under control.
State television said exports were continuing, however Aramco has yet to comment since the assault, which the Houthi rebels said involved 10 drones. Authorities have not said whether oil production or exports were affected.
The attacks occurred as Aramco accelerates plans for an initial public offering of the state oil giant to as early as this year, and follow earlier cross-border attacks on Saudi oil installations and on oil tankers in Gulf waters.
Today's attacks appeared to be the most brazen yet.
Saudi Arabia, leading a Sunni Muslim military coalition that intervened in Yemen in 2015 against the Houthis, has blamed regional rival Shi'ite Iran for previous attacks, which Tehran denies.
Riyadh accuses Iran of arming the Houthis, a charge denied by the group and Tehran.
State-run Ekhbariya TV, citing its correspondent, said there were no casualties, but there was no official statement.
A witness nearby said at least 15 ambulances were seen in the area and there was a heavy security presence around Abqaiq.