The US has reportedly identified locations in Iran from which drones and cruise missiles were launched against major Saudi oil facilities on Saturday.
A senior official told CBS News that the locations were in southern Iran, at the northern end of the Gulf.
Saudi air defences did not stop the drones and missiles because they were pointed southwards, to prevent attacks from Yemen, the official 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 attacked Saudi oil facilities 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.
Iranian President Hassan Rouhani previously called the attack a reciprocal act by the “Yemeni people”.
Oil prices soared 20% after the attacks but have since pared their gains. The international benchmark used by traders, Brent crude, is currently trading at around $65.34 (£52.32).
What has the US said?
President Donald Trump said on Monday it looked like the attacks had come from Iran, and US officials said they were gathering evidence to prove it.
Vice-President Mike Pence said on Tuesday the military was “ready to defend the interests of our allies”, and that Secretary of State Mike Pompeo was en route to Saudi Arabia “to discuss our response”.
One senior official told AFP news agency they would present their evidence at the UN General Assembly next week.
The unnamed official refused to give details about the attack, but insisted the US was certain they had come from Iranian soil.
In a televised address, Iran’s Supreme Leader Ayatollah Ali Khamenei dismissed talks “at any level” with the US, saying any dialogue could only happen if the US “takes back its words and repents” after withdrawing from the Iran nuclear deal.
Earlier this year, President Trump unilaterally pulled the US out of the landmark agreement that had limited Iran’s nuclear programme in exchange for the lifting of economic sanctions.
What happened in the attacks?
The attacks targeted Abqaiq – run by the Saudi state oil company, Aramco – and the Khurais oilfield.
Khurais is the closest of the targets to the Yemen border, but is still a considerable 770km (480 miles) away.
US officials said there were 19 points of impact on the targets, which could have come from a mix of drones and cruise missiles.
The Houthis have repeatedly launched rockets, missiles and drones at populated areas in Saudi Arabia, which have left at least four civilians dead.
The conflict in Yemen escalated in March 2015, when the Houthis took control of much of the west of the country and forced President Abdrabbuh Mansour Hadi to flee abroad.
Soon after, Saudi Arabia and its allies launched an air campaign in a bid to restore Mr Hadi’s government.
The UN says at least 7,290 civilians have died and 80% of the population – 24 million people – need humanitarian assistance or protection, including 10 million who rely on food aid to survive.
Iran’s Foreign Minister Javad Zarif previously said that “blaming Iran won’t end the disaster” in Yemen.