The precise targets and the extent of possible damage were unclear. Aramco did not immediately respond to a request for comment, according to Reuters.

Abqaiq, 60 km (37 miles) southwest of Dhahran in Saudi Arabia’s Eastern Province, contains the world’s largest oil processing plant. Khurais, 190 km further southwest, contains the country’s second largest oilfield.

Houthi Ansarullah fighters in Yemen have launched attacks over the border, hitting Shaybah oilfield with drones last month and two oil pumping stations in May. Both attacks caused fires but did not disrupt production.

The attacks are carried out in retaliation for the continued aggression of the Saudi-led coalition against Yemen.

Saudi Arabia and a number of its regional allies launched a devastating campaign against Yemen in March 2015, with the goal of bringing the government of former president Abd Rabbuh Mansur Hadi back to power and crushing the Houthi Ansarullah movement.

The US-based Armed Conflict Location and Event Data Project (ACLED), a nonprofit conflict-research organization, estimates that the war has claimed more than 91,000 lives over the past four and a half years.

The war has also taken a heavy toll on the country’s infrastructure, destroying hospitals, schools, and factories. The UN says over 24 million Yemenis are in dire need of humanitarian aid, including 10 million suffering from extreme levels of hunger.