The Iran-aligned Houthi movement that controls Yemen's capital claimed responsibility for the attack, which damaged the world's biggest crude oil processing plant.

Iran denied blame and said it was ready for "full-fledged war".

Two sources briefed on the operations of state oil company Saudi Aramco told Reuters it may take months for Saudi oil production to return to normal. Earlier estimates had suggested it could take weeks.

Oil prices surged by as much as 19 per cent before coming off peaks. The intraday jump was the biggest since the 1991 Iraqi invasion of Kuwait that prompted the Gulf War.

Prices eased after Trump announced that he would release US emergency supplies, and producers around the globe said there were enough stocks stored up to make up for the shortfall.

"There is reason to believe that we know the culprit, are locked and loaded depending on verification, but are waiting to hear from the Kingdom as to who they believe was the cause of this attack, and under what terms we would proceed!" Trump said on Twitter on Sunday.

While Iran has denied blame for the attacks, its Yemeni allies have promised more strikes to come. Houthi military spokesman Yahya Sarea said the group carried out Saturday's pre-dawn attack with drones, including some powered by jet engines.

"We assure the Saudi regime that our long arm can reach any place we choose and at the time of our choosing," Sarea tweeted. "We warn companies and foreigners against being near the plants that we struck because they are still in our sights and could be hit at any moment."

US officials say they believe that the attacks came from the opposite direction, possibly from Iran itself rather than Yemen, and may have involved cruise missiles. Wherever the attacks were launched, however, they believe Iran is to blame.

"There's no doubt that Iran is responsible for this. No matter how you slice it, there's no escaping it. There's no other candidate," a US official said on Sunday on condition of anonymity.

Tension in the Gulf region dramatically escalated this year after Trump imposed severe US sanctions on Iran aimed at halting its oil exports altogther.

Foreign Ministry spokesman Abbas Mousavi called the US accusations of Iranian involvement in Saturday's attacks "unacceptable and entirely baseless".

China's foreign ministry said on Monday it was irresponsible to assign blame for the attack without conclusive facts. The British foreign secretary said a clearer picture was needed before discussing responsibility for a "wanton violation of international law".

The plant that was struck cleans crude oil of impurities, a necessary step before it can be exported and fed into refineries. The attack cut Saudi output by 5.7 million barrels a day, or around half.

A long outage would make markets subject to wild swings that could potentially destabilise the global economy. But big countries such as the United States and China have reserves designed to handle even a major outage over the short term.

Russian Energy Minister Alexander Novak told reporters there was enough oil in commercial stockpiles to cover the shortfall.

The US official who asked not to be identified said on Sunday there were 19 points of impact in the attack and evidence showed the launch area was west-northwest of the targets - not south from Yemen. Saudi officials had seen signs that cruise missiles were used in the attack, the official added.

Australian Associated Press