Trump has said he does not want war and is coordinating with Gulf and European states
US President Donald Trump ordered a major increase in sanctions on Iran on Wednesday as Saudi Arabia displayed remnants of drones and missiles it said were used in a crippling attack "unquestionably sponsored" by Tehran.
Trump gave no explanation in a Twitter post announcing the order, but it followed repeated US assertions that the Islamic Republic was behind Saturday's attack and came hours after Saudi Arabia said the strike was a "test of global will".
"I have just instructed the Secretary of the Treasury to substantially increase sanctions on the country of Iran!" he wrote.
Iran again denied involvement in the Sept. 14 raids, which hit the world's biggest crude processing facility and initially knocked out half of Saudi output. Responsibility was claimed by Yemen's Iran-aligned Houthi group.
In an attempt to support its assertion that Iran was responsible, Saudi Arabia displayed drone and missile debris it said was undeniable evidence of Iranian aggression.
A total of 25 drones and missiles were used in the attacks launched from Iran not Yemen, Defence Ministry spokesman Colonel Turki al-Malki told a news conference."The attack was launched from the north and unquestionably sponsored by Iran," he said, adding Iranian Delta Wing unmanned aerial vehicles (UAV) were used in addition to cruise missiles.
An investigation into where the attacks were launched from was still under way and the result would be announced at a later date, he said.
Proof of Iranian responsibility, and in particular firm evidence that it was launched from Iranian territory, could pressure Riyadh and Washington into a response. Both nations, however, were stressing the need for caution.
Trump has said he does not want war and is coordinating with Gulf and European states.
Saudi Arabia's Crown Prince Mohammed bin Salman said the strike was a "real test of the global will" to confront subversion of the international order.
His envoy to London, Prince Khalid bin Bander, told the BBC the attack was "almost certainly" Iranian-backed: "We're trying not to react too quickly because the last thing we need is more conflict in the region."
Iran dismissed the allegations.
"They want to impose maximum ... pressure on Iran through slander," Iranian President Hassan Rouhani said.
"We don't want conflict in the region ... Who started the conflict?" he added, blaming Washington and its Gulf allies for the war in Yemen.
Yemen's Houthi movement, an ally of Iran battling a Western-backed, Saudi-led coalition for more than four years, has claimed responsibility and said it used drones to assault state oil company Aramco's sites.
The attack exposed the vulnerability of Saudi Arabia's oil infrastructure and threw down a gauntlet to the United States, which wants to curb Tehran's influence in the region.
US Secretary of State Mike Pompeo was to meet Prince Mohammed in Jeddah on Wednesday to discuss the crisis before heading to the United Arab Emirates.
UN officials monitoring sanctions on Iran and Yemen were also heading to Saudi Arabia to investigate.
France, which is trying to salvage an international nuclear deal with Iran that Washington quit last year, said it wanted to establish the facts before reacting.
A US official told Reuters the strikes originated in southwestern Iran. Three officials said they involved cruise missiles and drones, indicating a higher degree of complexity and sophistication than initially thought.
The officials did not provide evidence or explain what US intelligence they were using for evaluating the attack, which cut 5% of global production.
Saudi Arabia said on Tuesday the 5.7 million barrels per day of output lost would be fully restored by the end of the month.
Oil prices fell after the Saudi reassurances, having surged more than 20% at one point on Monday - the biggest intra-day jump since the 1990-91 Gulf War. [O/R]
Saudi Arabia's finance minister told Reuters the attack had no impact on revenues and Aramco was continuing to supply markets without interruption.
US efforts to bring about a UN Security Council response looked unlikely to succeed as Russia and China have veto powers and were expected to shield Iran.
Russian President Vladimir Putin, who has offered to sell Riyadh defence systems, called for a "thorough and impartial" probe during a phone call with Prince Mohammed.
The assault exposed gaps in Saudi air defences despite billions spent on Western military hardware.
"The attack is like Sept. 11th for Saudi Arabia, it is a game changer," said one Saudi security analyst.
Already frayed US-Iran ties deteriorated further when Trump quit the nuclear pact and reimposed sanctions, severely hurting the Iranian economy. Iran has ruled out talks with Washington unless it returns to the pact.
Trump said he is not looking to meet Rouhani during a UN event in New York this month. Rouhani and his foreign minister may not attend the General Assembly at all unless US visas are issued in the coming hours, state media reported on Wednesday.
Washington and its Gulf allies want Iran to stop supporting regional proxies, including in Yemen, Iraq and Lebanon.
Despite years of air strikes against them, the Houthi movement boasts drones and missiles able to reach deep into Saudi Arabia, the result of an arms race since the Western-backed coalition intervened in Yemen in March 2015.
Iran's clerical rulers support the Houthis, who ousted Yemen's internationally recognised government from power in the capital Sanaa in late 2014. But Tehran denies it actively supports them with military and financial support.