WASHINGTON (AFP) - Iran's supreme leader Ayatollah Ali Khamenei approved last weekend's attack on Saudi Arabia's oil infrastructure, CBS News reported on Wednesday (Sept 18), citing an unnamed US official.

The report came after US Secretary of State Mike Pompeo, flying to the kingdom, called last Saturday's strikes which knocked out half of Saudi crude output an "act of war".

CBS, without giving specifics about the US official or how they obtained the information, said Ayatollah Khamenei approved the attack only on condition that it be carried out in a way to deny Iranian involvement.

US officials quoted said the most damning evidence against Iran is unreleased satellite photos showing Iran's Revolutionary Guard Corps making preparations for the attack at Iran's Ahvaz air base.

The significance of those photos became clear only later, the report said.

"We were caught completely off guard," the network quoted a US official as saying.

Iranian state media said Teheran had written to Washington through the Swiss embassy on Monday, denying any role in the attacks and warning it would respond to any action against it.

Teheran-supported Houthi rebels in the kingdom's southern neighbour Yemen have claimed responsibility, but both Washington and Riyadh have ruled that out.

A US official, speaking on condition of anonymity, earlier told AFP the US administration has concluded the attack involved cruise missiles from Iran, and said evidence would be presented at the UN General Assembly next week.

Vice-President Mike Pence reiterated President Donald Trump's comments that "we don't want war with anybody, but the United States is prepared".

Mr Trump last year pulled out of a hard-won deal between Teheran and international powers that froze Iran's nuclear programme in return for the lifting of sanctions.

Washington reimposed crippling economic penalties, including against Iran's vital oil sector, and Teheran began taking steps back from the deal.