Saudi Arabia, leading a coalition of Sunni Muslim countries that intervened in Yemen
in 2015 against the Houthis, has blamed regional rival Shi'ite Iran for previous attacks, which Tehran denies. Riyadh accuses Iran of arming the Houthis, a charge denied by the group and Tehran.
The extent of damage from the drone strikes in Abqaiq and Khurais provinces remains unclear. Nine hours after the pre-dawn attacks, Aramco has issued no statement and the authorities have not reported on casualties.
Abqaiq is located 60 km (37 miles) southwest of Aramco's Dhahran headquarters. It contains the world's largest oil processing plant, handling crude from the giant Ghawar field and for export to terminals Ras Tanura - the world's biggest offshore oil loading facility - and Juaymah. It also pumps westwards across the kingdom to Red Sea export terminals.
Khurais, 190 km further southwest, contains the country's second largest oilfield.
Many Western employees of Aramco live in Abqaiq. The U.S. Embassy in Riyadh said it was unaware of any injuries to Americans from the attacks.
FIRE AND SMOKE
Hours after the strike in Abqaiq, a Reuters witness nearby said fire and smoke were still visible. Earlier video footage verified by Reuters showed bright flames and thick plumes of smoke rising towards the dark pre-dawn sky. An emergency vehicle is seen rushing towards the site.
The Saudi interior ministry spokesman said Aramco industrial security teams fighting the fires since 0400 (0100 GMT) had managed to control them and stop their spread. He did not identify the source of the drones but said an investigation was underway.
The Houthis' military spokesman, without providing evidence, said the attacks had achieved direct hits on refineries at both sites, which are over 1,000 km from the Yemeni capital Sanaa, and pledged a widening of attacks on Saudi Arabia.
The Saudi-led coalition launched two air strikes on Yemen's northern Saada province, a Houthi stronghold, on Saturday, a Reuters witness said. The Houthi-run al Masirah TV said the warplanes targeted a military camp north of Saada city.
Tensions in the region have escalated in recent months after President Donald Trump pulled the United States out of an international
nuclear deal and extended economic sanctions on Iran.
The Houthis hit Shaybah oilfield last month and two oil pumping stations in May. Both attacks caused fires but did not disrupt production.
The coalition has responded with air strikes on Houthi targets in Sanaa and other areas held by the group, which controls most large urban centres in Yemen.
The violence is complicating U.N.-led peace efforts to ease tensions between the Houthis and Saudi Arabia to pave the way for political talks to end the war, which has killed tens of thousands of people and pushed millions to the brink of famine.
The Western-backed coalition intervened in Yemen
to try to restore the internationally recognised government ousted from power in the capital Sanaa by the Houthis in late 2014.
The Yemen conflict is widely seen as a proxy war between Saudi Arabia and Iran. The Houthis deny being puppets of Tehran and say they are fighting a corrupt system.