DUBAI, United Arab Emirates >> Drones launched by Yemen’s Houthi rebels attacked the world’s largest oil processing facility in Saudi Arabia and another major oil field today, 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 Persian Gulf amid an escalating crisis 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 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 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 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.

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 recognized Yemeni government.

The U.S. Embassy in Riyadh said it was unaware of any injuries to Americans. Saudi Aramco employs a number of U.S. citizens, some of whom live in guarded compounds in the kingdom near the site.

“These attacks against critical infrastructure endanger civilians, are unacceptable, and sooner or later will result in innocent lives being lost,” U.S. Ambassador John Abizaid, a former Army general, said.

Saudi Aramco describes its Abqaiq oil processing facility in Buqyaq as “the largest crude oil stabilization plant in the world.”

The facility processes sour crude oil into sweet crude, then transports it onto transshipment points on the Persian Gulf and the Red Sea or to refineries for local production. Estimates suggest it can process up to 7 million barrels of crude oil a day. By comparison, Saudi Arabia produced 9.65 million barrels of crude oil a day in July.

The plant has been targeted in the past by militants. Al-Qaida-claimed suicide bombers tried but failed to attack the oil complex in February 2006.

The Khurais oil field is believed to produce over 1 million barrels of crude oil a day. It has estimated reserves of over 20 billion barrels of oil, according to Aramco.

There was no immediate impact on global oil prices as markets were closed for the weekend. Benchmark Brent crude had been trading at just above $60 a barrel.

While Saudi Arabia has taken steps to protect itself and its oil infrastructure, analysts had warned that Abqaiq remained vulnerable. The Rapidan Energy Group, a Washington-based advisory group, warned in May that “a successful attack could lead to a monthslong disruption of most Saudi production and nearly all spare production.” It called Abqaiq, close to the eastern Saudi city of Dammam, “the most important oil facility in the world.”

The Washington-based Center for Strategic and International Studies separately issued its own warning just last month.

“Though the Abqaiq facility is large, the stabilization process is concentrated in specific areas . including storage tanks and processing and compressor trains — which greatly increases the likelihood of a strike successfully disrupting or destroying its operations,” the center said.

The war 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 the U.S.-based Armed Conflict Location & Event Data Project, or ACLED, which tracks the conflict.

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 U.N., the West and Gulf Arab nations say Tehran does.

The rebels have flown drones into the radar arrays of Saudi Arabia’s Patriot missile batteries, according to Conflict Armament Research, disabling them and allowing the Houthis to fire ballistic missiles into the kingdom unchallenged. The Houthis launched drone attacks targeting Saudi Arabia’s crucial East-West Pipeline in May as tensions heightened between Iran and the U.S. In August, Houthi drones struck Saudi Arabia’s Shaybah oil field, which produces some 1 million barrels of crude oil a day.