Drones launched by Yemen's Houthi rebels have attacked the world's largest oil processing facility in Saudi Arabia and another major oilfield, sparking huge fires.
It remains unclear whether anyone was injured at the Abqaiq oil processing facility and the Khurais oilfield, or what effect the assault will have on global oil production.
Rising smoke from fires at the sites could be seen by satellites in space.
The attack by the Iranian-backed Houthis in the war against a Saudi-led coalition comes after weeks of similar drone assaults on the kingdom’s oil infrastructure, but none of the earlier strikes appear to have caused the same amount of damage.
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The attack will likely heighten tensions further across the wider Persian Gulf amid a confrontation between the US and Iran over its unravelling nuclear deal with world powers.
Initial reports of the assault came in online videos of giant fires at the Abqaiq facility, some 205 miles north-east of the Saudi capital, Riyadh.
Machine gun fire could be heard in several clips alongside the day’s first Muslim call to prayers, suggesting security forces tried to bring down the drones in the darkness just before dawn.
In daylight, Saudi state television aired a segment with its local correspondent near a police checkpoint, with a thick plume of smoke visible behind him.
The fires began after the sites were targeted by drones, the interior ministry said in a statement carried by the state-run Saudi Press Agency. An investigation is under way.
Saudi Aramco, the state-owned oil giant, did not respond to questions. The kingdom hopes soon to offer a sliver of the company in an initial public offering.
In a short address aired by the Houthi's Al-Masirah satellite news channel, military spokesman Yahia Sarie said the rebels launched 10 drones in a coordinated attack on the sites after receiving "intelligence" support from those inside the kingdom.
He warned that attacks by the rebels would only get worse if the war continues.
“The only option for the Saudi government is to stop attacking us,” Mr Sarie said.
The rebels hold Yemen's capital, Sanaa, and other territory in the Arab world's poorest country.
Since 2015, a Saudi-led coalition has fought to reinstate the internationally recognised Yemeni government.
The plant has been targeted in the past by militants. Suicide bombers claimed by al-Qaeda failed in an attempt to attack the oil complex in February 2006.
The Khurais oil field is believed to produce more than one million barrels of crude oil a day. It has estimated reserves of more than 20 billion barrels of oil, according to Aramco.
The war in Yemen has become the world's worst humanitarian crisis. The violence has pushed Yemen to the brink of famine and killed more than 90,000 people since 2015, according to US figures.
Since the start of the Saudi-led war, Houthi rebels have been using drones in combat.
The first appeared to be off-the-shelf, hobby-kit-style drones. Later, versions nearly identical to Iranian models turned up. Iran denies supplying the Houthis with weapons, although the UN, the West and Gulf Arab nations contradicted this. - AP