Drone attacks have set alight two major oil facilities run by the state-owned company Aramco in Saudi Arabia, state media say.
Footage showed a huge blaze at Abqaiq, site of Aramco's largest oil processing plant, while a second drone attack started fires in the Khurais oilfield.
The fires are now under control at both facilities, state media said.
A spokesman for the Iran-aligned Houthi group in Yemen said it had deployed 10 drones in the attacks.
The military spokesman, Yahya Sarea, told al-Masirah TV, which is owned by the Houthi movement and is based in Beirut, that further attacks could be expected in the future.
He said Saturday's attack was one of the biggest operations the Houthi forces had undertaken inside Saudi Arabia and was carried out in "co-operation with the honourable people inside the kingdom".
Saudi Arabia is said to be shutting down around half of its oil output, the Wall Street Journal reports.
Officials have not yet commented on who they think is behind the attacks.
"At 04:00 (01:00 GMT), the industrial security teams of Aramco started dealing with fires at two of its facilities in Abqaiq and Khurais as a result of... drones," the official Saudi Press Agency reported.
"The two fires have been controlled."
There have been no details on the damage but Agence France-Presse quoted interior ministry spokesman Mansour al-Turki as saying there were no casualties.
Abqaiq is about 60km (37 miles) south-west of Dhahran in Saudi Arabia's Eastern Province, while Khurais, some 200km further south-west, has the country's second largest oilfield.
Saudi security forces foiled an attempt by al-Qaeda to attack the Abqaiq facility with suicide bombers in 2006.
An attack method open to all
Jonathan Marcus, BBC defence and diplomatic correspondent
This latest attack underlines the strategic threat posed by the Houthis to Saudi Arabia's oil installations.
The growing sophistication of the Houthis' drone operations is bound to renew the debate as to where this capability comes from. Have the Houthis simply weaponised commercial civilian drones or have they had significant assistance from Iran?
The Trump administration is likely to point the finger squarely at Tehran, but experts vary in the extent to which they think Iran is facilitating the drone campaign.
The Saudi Air Force has been pummelling targets in Yemen for years. Now the Houthis have a capable, if much more limited, ability to strike back. It shows that the era of armed drone operations being restricted to a handful of major nations is now over.
Drone technology - albeit of varying degrees of sophistication - is available to all; from the US to China, Israel and Iran... and from the Houthis to Hezbolllah.
Markets await news from key facilities
Analysis by BBC business correspondent Katie Prescott
Aramco ranks as the world's largest oil business and these facilities are significant.
The Khurais oilfield produces about 1% of the world's oil and Abqaiq is the company's largest facility - with the capacity to process 7% of the global supply. Even a brief or partial disruption could affect the company, and the oil supply, given their size.
But whether this will have an impact on the oil price come Monday will depend on just how extensive the damage is. Markets now have the weekend to digest information from Aramco and assess the long-term impact.
According to Richard Mallinson, geopolitical analyst at Energy Aspects, any reaction on Monday morning is likely to be muted, as markets are less worried about supply than demand at the moment, due to slower global economic growth and the ongoing trade war between the US and China.
However, there are concerns that escalating tensions in the region could pose a broader risk, potentially threatening the fifth of the world's oil supply that goes through the critical Strait of Hormuz.
Who are the Houthis?
The Iran-aligned Houthi rebel movement has been fighting the Yemeni government and a Saudi-led coalition.
Yemen has been at war since 2015, when President Abdrabbuh Mansour Hadi was forced to flee the capital Sanaa by the Houthis. Saudi Arabia backs President Hadi, and has led a coalition of regional countries against the rebels.
The coalition launches air strikes almost every day, while the Houthis often fire missiles into Saudi Arabia.
Mr Sarea, the Houthi group's military spokesman, told al-Masirah that operations against Saudi targets would "only grow wider and will be more painful than before, so long as their aggression and blockade continues".
Houthi fighters were blamed for drone attacks on the Shaybah natural gas liquefaction facility last month and on other oil facilities in May.
There have been other sources of tension in the region, often stemming from the rivalry between Saudi Arabia and Iran.
Saudi Arabia and the US both blamed Iran for attacks in the Gulf on two oil tankers in June and July, allegations Tehran denied.
In May, four tankers, two of them Saudi-flagged, were damaged by explosions within the UAE's territorial waters in the Gulf of Oman.
Saudi Arabia and then US National Security Adviser John Bolton blamed Iran. Tehran said the accusations were "ridiculous".
Tension in the vital shipping lanes worsened when Iran shot down a US surveillance drone over the Strait of Hormuz in June, leading a month later to the Pentagon announcing the deployment of US troops to Saudi Arabia.