ITV News Correspondent Emma Murphy explains the US thinking
The United States of America says missiles and drones used in the weekend attacks on Saudi Arabian oil installations originated in Iran.
US Secretary of State, Mike Pompeo, who already suspected Iran, is on his way to the Middle Eastern country to help manage the situation.
The attack early on Saturday struck a Saudi oil field and the world's largest crude oil processing plant in the kingdom's eastern region, taking out 5.7 million barrels of crude oil production per day for the country, or about 5% of the world's daily production.
Houthi rebels in Yemen - where Saudi Arabia backs the government in the country's civil war - say they launched the missiles, but on Tuesday, senior US officials claimed the missiles or drones were launched from southern Iran.
Saudi air defences did not stop the drones and missiles because they were pointed southwards, to prevent attacks from Yemen, they added.
Iran denies involvement in the attacks, which disrupted global oil supplies.
Yemen's Iran-aligned Houthi rebels said they had launched the drones that struck the Abqaiq oil processing plant, the world's largest, and the Khurais oilfield.
They have launched attacks into Saudi Arabia before, but US officials said on Sunday they believed the drones and missiles did not originate from the south or south-west, and instead were launched from the north or north-west.
The claims came as Prime Minister Boris Johnson held telephone talks with Saudi Arabia's crown prince in the wake of the attacks.
A Number 10 spokesperson said: "The Prime Minister spoke to Saudi Crown Prince Mohammed bin Salman this evening to discuss the recent attacks on Aramco oil facilities.
"He expressed the UK's condemnation of the attacks and said the UK stands by Saudi Arabia and is committed to the country's security.
"The two leaders noted the need to establish the facts of what happened and the Prime Minister reiterated the importance of a collective response.
"He encouraged the Crown Prince to continue working with international partners."
Saudi Arabia's energy minister said on Tuesday more than half of the country's daily crude oil production that was knocked out by an attack had been recovered and that production capacity at its targeted plants would be fully restored by the end of the month.
"Where would you find a company in this whole world that went through such a devastating attack and came out like a phoenix?" Prince Abdulaziz bin Salman said about the state-owned Saudi Aramco, which was the target of the attacks.
His question to reporters, many of them Saudi, drew applause.
The state-run company's ability to quickly recover from an attack of this magnitude on its most important processing facility suggests that the feared price rises across the globe may be lessened.
Prince Abdulaziz said Aramco will honor its commitments to its customers this month by drawing from its reserves of crude oil and offering additional crude production from other oil fields.