Donald Trump announced new sanctions against Iran on Wednesday night as he sought to build consensus with allies over responding to the attack on two Saudi Arabian oil facilities.
Mr Trump talked to Boris Johnson and agreed the need for a "united diplomatic response". Mike Pompeo, the US secretary of state, was dispatched to Saudi Arabia for talks.
The Saudi defence ministry held a briefing before a backdrop of burnt pieces of cruise missiles and drones, declaring that Iran "unquestionably sponsored" the attack.
Iran has denied involvement and Houthi rebels in Yemen, whom it backs, have claimed responsibility. An adviser to Hassan Rouhani, the Iranian president, said Saudi Arabia "knows nothing".
Mr Trump said he was considering all possible responses, including "the ultimate option" of war, adding: "There's plenty of time to do some dastardly things." Mr Pompeo called the attack "an act of war".
Mr Trump's decision to "substantially increase" sanctions on Iran, details of which are yet to be released, makes a meeting with Mr Rouhani at the UN this week unlikely. Iran threatened on Wednesday to pull out of the trip.
At the Saudi briefing, defence mnistry spokesman Col Turki Al-Malki said the attack did not come from Yemen, as Iran has claimed, but from north of Saudi Arabia.
He said that Iran “sponsored” the attack but fell short of alleging it had been launched from Iranian ground, noting the search to pinpoint the exact site continued.
Mr Trump has repeatedly said he does not want a military conflict with Iran and a Saudi official pointedly did not promise a military response when asked in their press conference.
The United Nations is now being drawn into the stand-off, with the Saudis promising the share what it has found with international investigators.
French President Emmanuel Macron's office announced experts from his nation would travel to Saudi Arabia to help shed light on the “origin and methods” of the attack.
The Saudi briefing, held before live TV cameras, escalated the stand-off between Saudi Arabia and Iran over Saturday’s attack on its Abqaiq oil facility and Khurais oilfield.
Eighteen drones and seven cruise missiles were launched in the assault, Mr Malki said, with three missiles failing to make their targets.
He said the cruise missiles had a range of 700 kilometers (435 miles), meaning they could not have been fired from inside Yemen.
I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!— Donald J. Trump (@realDonaldTrump) September 18, 2019
Mr Malki played surveillance video he said showed a drone coming in from the north of Saudi Arabia. That raises the possibility that it was fired from Iran or Iraq.
"The attack was launched from the north and was unquestionably sponsored by Iran," Mr Malki told reporters.
He added: "This attack did not originate from Yemen, despite Iran's best effort to make it appear so."
During the briefing Mr Malki declined to promise a military response once the exact launch site for the attack is located, saying only that there would be “accountability”.
Hesameddin Ashena, an adviser to Hassan Rouhani, the Iranian president, hit back at the claims on Twitter.
“The press conference proved that Saudi Arabia knows nothing about where the missiles and drones were made or launched from and failed to explain why the country's defence system failed to intercept them," he wrote.
The fallout from the Saudi oil attack, which had sent the world oil price soaring, has disrupted Mr Trump’s hopes of meeting Mr Rouhani at the United Nations in New York next week.
For months Mr Trump has talked up the chance of a handshake - the first between US and Iranian leaders for decades - amid reports he is seeking a deal with Tehran before the 2020 US presidential election.
But earlier this week Iran’s supreme leader, Ayatollah Ali Khamenei, said there would be no talks “at any level” and on Wednesday Mr Trump announced new sanctions.
He tweeted: “I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!” No more details were provided, with Mr Trump saying they would come in the next 48 hours.
Iranian state media reported that Mr Rouhani may not even attend the UN gathering at all, claiming that Iran's preparatory delegation had been denied visas.
Speaking to reporters in California on Wednesday, Mr Trump did not rule out launching a military strike in retaliation to last week’s attack.
“There are many options. There’s the ultimate option and there are options that are a lot less than that,” Mr Trump said. Asked to clarify, he said the ultimate option was war.
Mr Trump also talked to Boris Johnson, the Prime Minister, yesterday. A Downing Street spokesman said: “They condemned the attacks and discussed the need for a united diplomatic response from international partners.”