Drones launched by Yemen’s Houthi rebels attacked the world’s largest oil processing facility in Saudi Arabia and another major oilfield Saturday, sparking huge fires at a vulnerable chokepoint for global energy supplies.

It remained unclear hours later whether anyone was injured at the Abqaiq oil processing facility and the Khurais oil field or what affect the assault would have on oil production. Rising smoke from the 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 appeared to have caused the same amount of damage. The attack likely will heighten tensions further across the wider Persian Gulf amid a confrontation between the U.S. and Iran over its unraveling nuclear deal with world powers.

First word of the assault came in online videos of giant fires at the Abqaiq facility, some 330 kilometers (205 miles) northeast 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, 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. It said an investigation was underway.

Saudi Aramco, the state-owned oil giant, did not respond to questions from The Associated Press. 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 their 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,” Sarie said.

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