Drones attacked the world’s largest oil processing facility in Saudi Arabia and an oilfield operated by Saudi Aramco early on Saturday, the kingdom’s Interior Ministry said, sparking a huge fire at a processor crucial to global energy supplies.

No one immediately claimed responsibility for the attacks in Buqyaq and the Khurais oil field, though Yemen’s Houthi rebels previously launched drone assaults deep inside of the kingdom.

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 into the attack was underway.

Saudi Aramco, the state-owned oil giant, did not immediately respond to questions from The Associated Press. The kingdom hopes soon to offer a sliver of the company in an initial public offering.

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

The facility processes sour crude oil into sweet crude, then later transports onto transshipment points on the Persian Gulf and the Red Sea. Estimates suggest it can process up to 7 million barrels of crude oil a day.

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.

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

Buqyaq is some 330 kilometers northeast of the Saudi capital, Riyadh.While no group immediately claimed the attacks, suspicion immediately fell on Yemen’s Houthi rebels.

A Saudi-led coalition has been battling the rebels since March 2015. The Iranian-backed Houthis hold Yemen’s capital, Sanaa, and other territory in the Arab world’s poorest country.