The drone strikes on plants in the heartland of Saudi Arabia’s oil industry, including the world’s biggest petroleum processing facility, were expected to send oil prices up $5-10 per barrel on Monday as tensions rise in the Middle East.

Yemen’s Houthi group claimed responsibility for Saturday’s attacks that knocked out more than half of Saudi oil output or more than 5% of global supply, but US Secretary of State Mike Pompeo said the assault was the work of Iran, a Houthi ally, Reuters reported.

A spokesperson from Iran's foreign ministry denied the accusation that Tehran was behind the attacks, explaining that "blind accusations and inappropriate comments in a diplomatic context are incomprehensible and meaningless."

Iraq also rejected the accusations that its territory was used to launch the attacks.

State oil giant Saudi Aramco said the attack cut output by 5.7 million barrels per day, at a time when Aramco is trying to ready itself for what is expected to be the world’s largest share sale.

A source close to the matter told Reuters the return to full oil capacity could take “weeks, not days.”

Traders and analysts said crude may spike to as high as $100 if Riyadh fails to quickly bring back supply.

Meanwhile, the EU warned that Saturday’s attack posed a real threat to regional security.

Saudi Arabia, the world’s top oil exporter, ships more than 7 million barrels of oil to global destinations every day.