Saudi Arabia’s oil production has been severely disrupted by drone attacks on two major oil facilities run by state-owned company Aramco, reports say.
Sources quoted by Reuters and WSJ said the strikes had reduced production by five million barrels a day – nearly half the kingdom’s output.
The fires are now under control at both facilities, Saudi state media say.
A spokesman for the Houthi rebel group in Yemen said it had deployed 10 drones in the attacks.
The Saudis lead a military coalition backing Yemen’s government, while Iran backs the Houthi rebels.
The Houthi 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”.
TV 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.
United Nations envoy Martin Griffiths described the attacks as “extremely worrying” in a statement in which he called on all parties in the Yemen conflict to exercise restraint.
Saudi officials have yet to comment on who they think is behind the attacks.
“At 04:00 (local time), 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 AFP news agency 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.
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.
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.