The drone attacks on Saudi Arabia's Aramco oil plants on Saturday (September 14) may knock out more than half of the country's oil output.
The state-run company said the strikes mean production will be cut by 5.7 million barrels per day.
That's more than 5% of the world's crude supply.
And all of this comes at a time when Aramco is gearing up for a stock market listing.
But stocks fell sharply on Sunday (September 15), and the decline has extended a losing spree.
Which in recent weeks have been hit by expensive valuations, weak oil prices, and concerns about the economic outlook.
Aramco gave no timeline for when output would resume.
A source told Reuters the return to full oil capacity could take "weeks, not days."
Yemen's Houthis, which are backed by Iran, said they were behind the attacks on Al Masirah TV on Saturday.
But U.S. Secretary of State Mike Pompeo accused Iran of being directly responsible.
He says there's no evidence the attacks came from the Houthis themselves.
Iran have dismissed the claim.
Pompeo's statement follows Western accusations of Iranian involvement in attacks on oil tankers earlier in the summer.