Saudi Arabia's oil production and exports have been disrupted by drone attacks on two Aramco facilities on Saturday, one of the world's largest oil refineries, three sources familiar with the matter told Reuters.
One source said the attacks would affect the production of five million barrels of oil per day, about half the current production of the Kingdom.
Aramco operates the world's largest oil refinery and crude processing plant in Abqaiq in the eastern region, and the refining capacity of the refinery exceeds seven million barrels of crude oil per day.
A Houthi drone attack in Yemen on Saturday set fire to two oil facilities belonging to Saudi oil giant Aramco, the third such attack in five months on its facilities.
The Saudi Interior Ministry said in a statement reported by the Saudi news channel that the industrial security teams in Aramco were able to control two fires that broke out in the two factories of the company after they were targeted by drones.
Songsters posted videos showing thick smoke in the Abqaiq sky after the attack.
Saudi authorities have beefed up security around the targeted sites and have prevented journalists from approaching to see the extent of damage at the oil sites from which smoke plumes were rising.
Abqaiq, 150 km east of Riyadh, is home to the world's largest oil refinery, and Khurais, 190 km southwest of Dhahran, is the world's second-largest oilfield.
The Houthis, in the words of their military spokesman Brigadier General Yahya Sari, said the attack was carried out by 10 drones and that the targeting was direct and accurate.
He added that the bank of their goals in Saudi Arabia is expanding day by day, and that there is no solution for Riyadh only to stop "aggression and siege."
The Houthis adopted the previous two attacks in May and August on Aramco facilities in the kingdom, which is leading a military alliance that has been waging an air campaign in Yemen since 2015 against the Houthis.
US Ambassador to Saudi Arabia John Abizaid on Saturday condemned the drone attack on two Saudi Aramco oil facilities, saying the attacks were "unacceptable behavior."
"The United States strongly condemns the attacks carried out today by drones on two oil facilities in Abqaiq province and Khurais migration," Abizaid was quoted by the embassy account as saying.
"These attacks on vital infrastructure that endanger civilians are unacceptable and, sooner or later, will result in the loss of innocent lives," he said.