Saudi Arabia raced on Sunday to restart operations at oil plants hit by drone attacks which slashed its production by half, as Iran dismissed US claims it was behind the assault.
The Tehran-backed Huthi rebels in neighbouring Yemen, where a Saudi-led coalition is bogged down in a five-year war, have claimed Saturday's strikes on two plants owned by state giant Aramco.
But 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 a foreign ministry spokesman said: "Such fruitless and blind accusations and remarks are incomprehensible and meaningless."
"It is now clear that Saudi and other Gulf oil facilities are vulnerable to this kind of attack, which means that the geopolitical risk premium for oil needs to rise."
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