Iran has threatened a crushing response to any military strike after it was blamed for attacks on Saudi oil sites, as US President Donald Trump said he has ordered increased sanctions on the Middle Eastern nation.
In a letter sent to the US via the Swiss embassy, which represents US interests in Iran, Tehran said it "denies and condemns claims" by US officials that Tehran was behind the attacks.
"It was also emphasised in the letter that in case of any aggression against Iran, that action will face an immediate response from Iran and the response won't be limited to its source," the state news agency IRNA reported.
The Etemad daily newspaper described US accusations of Iranian responsibility for the Saudi attacks as part of Washington's stated "maximum pressure" policy to isolate Iran.
"[We are] fully prepared to surprise aggressors through a crushing and comprehensive answer to possible evil actions," Etemad quoted Secretary of Iran's Supreme National Security Council Ali Shamkhan as saying.
Trump said on Wednesday he had ordered Treasury Secretary Steven Mnuchin to "substantially increase sanctions" imposed on Iran, but did not give additional details.
A US official told Reuters that projectiles that hit Saudi oil facilities on Saturday came from southwest Iran.
Three officials said they involved cruise missiles and drones, indicating a higher degree of complexity and sophistication than initially thought.
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The Iran-aligned Houthi group in Yemen, to Saudi Arabia's south, claimed responsibility for the attack. But Saudi Arabia said it would produce evidence on Wednesday (local time) linking Iran, to the northeast across the Gulf, to the assault.
Iranian President Hassan Rouhani said the attack had been mounted by Houthis in retaliation for Yemen's war, which he blamed on the US and a Saudi-led military coalition.
"We don't want conflict in the region ... who started the conflict? Not Yemenis. It was Saudi Arabia, the Emirates, America, certain European countries and the Zionist regime which started the war in this region," he said in a video carried by state media.
President Donald Trump has stepped up sanctions against Iran since last year when he exited a 2015 nuclear pact between Iran and six world powers.
He reimposed sanctions that were lifted under the deal in return for Iran curbing its nuclear program.
In response, Iran has gradually scaled back its commitments to limit uranium enrichment activity under the pact and plans to further breach it if European signatories fail to keep their promises to shield Iran's economy from US penalties.
Saudi Arabia said on Tuesday that more than half of its oil production, knocked out by the attacks, had been recovered and that production capacity at its targeted plants would be fully restored by the end of September.
Iranian Oil Minister Bijan Zanganeh said the attacks on a Saudi oil field and the world's largest crude processing plant would affect oil prices everywhere but only in the short-term.
The US Secretary of State will meet with Saudi Arabia's crown prince in Jeddah on Wednesday to discuss the attack and coordinate efforts against "Iranian aggression".
The US Mission to the United Arab Emirates said in a Twitter posting that Mike Pompeo "will meet with Saudi Crown Prince Mohammed bin Salman to discuss the recent attack on the Kingdom's oil facilities and coordinate efforts to counter Iranian aggression in the region".
Pompeo will then travel to the UAE capital Abu Dhabi to meet with its crown prince, Sheikh Mohammed bin Zayed, and discuss regional and bilateral issues, it said.
