Mike Pompeo said there was no evidence the attacks had been launched from Iraq.

US Secretary of State Mike Pompeo denounced strikes on Saudi Arabia's oil infrastructure as an "act of war" Wednesday, as Riyadh unveiled new evidence it said showed the assault was "unquestionably" sponsored by arch-foe Iran.

The comments raise the risk of a wider conflict in the tinderbox Gulf region after the weekend strikes on the heart of the kingdom's oil industry knocked out half its production.

"This was an Iranian attack," Pompeo told reporters on his plane before landing in the western city of Jeddah, calling it "an act of war".

His comment came as Saudi Arabia displayed what it said were fragments of 25 drones and cruise missiles fired at two facilities in the country's east, leaving them engulfed in flames.

"The attack was launched from the north and unquestionably sponsored by Iran," defence ministry spokesman Turki al-Maliki said.

"We are working to know the exact launch point."

But Maliki would not be drawn on whether Saudi Arabia believed Iran would ultimately be found to be the culprit, only saying they were confident they would find out from where the weapons were fired.

Pompeo said there was no evidence the attacks had been launched from Iraq -- caught between its two main sponsors, Tehran and Washington -- amid media speculation this was the case.

Diplomats at the United Nations said experts were expected in the kingdom to lead an international inquiry.

Pompeo, who has directly blamed Iran for the strikes, was due to hold talks Wednesday with Saudi leadership as he arrived in Jeddah to weigh with the US ally a response to the strike that roiled global energy markets.

Yemen's Iran-backed Huthi rebels, who have claimed Saturday's strikes, vowed meanwhile they had the means to hit "dozens of targets" in the United Arab Emirates.

'Prefer not to meet'

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

"As the president said, we don't want war with anybody, but the United States is prepared," Vice President Mike Pence said Tuesday.

President Donald Trump -- who has already re-imposed sanctions that have crippled Iran's economy -- Wednesday promised to "substantially increase" the measures, winning quick praise from Riyadh.

He said details would be given within 48 hours.

Iran's Foreign Minister Mohammad Javad Zarif, himself under US sanctions since July 31, described the measures as "illegal" and "inhuman".

Trump's move was an "admission that (the) US is DELIBERATELY targeting ordinary citizens," he said on Twitter.

The apparent hardening of the US position came as Iran's supreme leader Ayatollah Ali Khamenei ruled out negotiations with Washington "at any level".

That appeared to nix hopes for a dramatic meeting between Trump and his Iranian counterpart Hassan Rouhani at the United Nations next week.

Speaking to reporters on Air Force One, Trump said he too had cooled on what had always seemed to be a diplomatic longshot.

"I prefer not meeting him," he said.

Late Wednesday, the United States still had not issued Rouhani and his delegation with visas to attend the meeting in New York, Iranian state media said.

Yemen's Huthi rebels, who are locked in a prolonged conflict with a Saudi-led military coalition, claimed responsibility for Saturday's attacks, which took out six percent of global supplies.

But Riyadh and Washington have both ruled that out.

"Despite Iran's efforts to make it appear so" they did not originate from Yemen, Maliki said, adding the strike was beyond the capabilities of the militia -- who have however mounted dozens of smaller attacks on Saudi territory.

"The precision impact of the cruise missile indicated advance capability beyond (Iranian) proxy capacity," he said.

They had also struck from a direction that ruled out Yemen as a source, he added.

Oil prices rocked

Observers say the Saudi experience in Yemen, where despite their vast firepower, they have failed to subdue the ragtag but highly motivated militia, has made Riyadh circumspect about wading into another conflict.

"I certainly hope we're not" going to have another war, Riyadh's ambassador to London Prince Khalid bin Bandar told the BBC.

"We are trying not to react too quickly because the last thing we need is more conflict in the region," he said.

Iran has claimed the Huthis were responsible for Saturday's attack, and Rouhani said Wednesday the rebels had done so as a "warning" about a possible wider war in response to the Saudi-led intervention in Yemen.

State media said Tehran had written to Washington through the Swiss embassy on Monday, denying any role in attacks on Saudi oil installations and warning it would respond to any action.

Trump's administration is considering responses to the latest attack, including a cyber attack or a physical strike on Iranian oil infrastructure or its Revolutionary Guards, NBC News reported, citing unnamed US officials.

Oil prices have see-sawed since the attacks, with record gains Monday followed by a tumble Tuesday as the Saudi assurances on supply soothed the markets.