US secretary of state Mike Pompeo (left) is greeted by Crown Prince Mohammed bin Salman in Jeddah, September 18 2019. Picture: BANDAR AL-JALOUD/SAUDI ROYAL PALACE/AFP
US secretary of state Mike Pompeo (left) is greeted by Crown Prince Mohammed bin Salman in Jeddah, September 18 2019. Picture: BANDAR AL-JALOUD/SAUDI ROYAL PALACE/AFP

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

The comments raise the risk of a wider conflict in the Gulf region after the weekend strikes on the heart of Saudi Arabia's oil industry knocked out half its production, rattling energy markets.

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

"This is an attack of a scale we've just not seen before."

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, engulfing them in flames.

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

Iran-backed Houthi rebels in Yemen have claimed responsibility, but both Washington and Riyadh have ruled that out.

"We are working to know the exact launch point," Maliki said.

Pompeo said there was no evidence for media reports the attacks had been launched from Iraq.

Diplomats at the UN said experts were expected in Saudi Arabia to lead an international inquiry.

Pompeo met Saudi Crown Prince Mohammed bin Salman in Jeddah to discuss a response to the strike, which took out 5% of global supplies.

Meanwhile, the Houthis threatened to hit "dozens of targets" in the United Arab Emirates, part of a Saudi-led coalition against the rebels.

'We don't want war' 

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

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

Trump, who has already re-imposed sanctions that have crippled Iran's economy, promised on Wednesday to "substantially increase" the measures, winning quick praise from Riyadh. He said details would be given within 48 hours.

Iranian foreign minister Mohammad Javad Zarif, under US sanctions since July 31, described the measures as "illegal" and "inhuman".

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

Trump has ruled out a meeting with Iranian counterpart Hassan Rouhani at the UN in October.

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

Maliki said Saturday's attack did not originate from Yemen, where Saudi Arabia is locked in a prolonged conflict with the Houthis, "despite Iran's efforts to make it appear so".

He said 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 advanced capabilities beyond those of the Houthis, he added.

Iran has backed Houthi claims of being behind the attack, and Rouhani said on Wednesday it was a rebel "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 installations and warning it would respond to any action against it.

Trump's administration is considering responses 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 on Monday followed by a tumble Tuesday as the Saudi assurances on supplies soothed the markets.