(CNN) - Yemen's Houthi rebels have attacked Saudi Arabia's oil facilities with drones, the Houthi-run Al-Masirah news agency said Saturday.
A Yemen armed forces spokesman was quoted by the agency as saying the Houthis successfully carried out a "large-scale" operation with 10 drones targeting Saudi Aramco oil facilities in Abqaiq and Khurais.
The Saudi interior ministry said the drone attacks caused fires at two facilities of the state-owned oil company. In a statement posted on Twitter, the ministry said the fires were under control and that authorities are still investigating the incident.
The Yemen armed forces spokesman said the operation was a "legitimate and natural" response to "the enemy's aggression and blockade" of Yemen.
Saudi Arabia has been leading a military campaign to quash the Houthi rebels in Yemen since March 2015. The conflict is widely seen as a proxy war between the Saudis and Iran, which has been backing the Houthis.
"We promise the Saudi regime that the next operation will be wider and more painful if the blockade and aggression continues," the spokesman added.
The Saudi-led coalition battling the Houthi rebels said in a statement it's investigating the attacks and would confront "terrorist" threats to global energy security.
One of the facilities, at Abqaiq, is the company's largest oil-processing facility and the largest crude oil stabilization plant in the world, according to the oil company, Aramco.
Saudi Arabia produces approximately 10% of the total global supply of 100 million barrels per day.
People with knowledge of Saudi's oil operations told CNN that capacity at Aramco's oil facilities is "down at this stage by 5 million barrels a day," meaning 5% of the daily global oil is affected.
Aramco "hopes to have that capacity restored within days," the source said.
The International Energy Agency is closely "monitoring the situation," the IEA said in a tweet. For now, "markets are well supplied with ample commercial stocks," the IEA said.
US President Donald Trump called Saudi Crown Prince Mohammad bin Salman on Saturday to offer his support for the country's self-defense, White House Deputy Press Secretary Judd Deere said in a statement.
"The United States strongly condemns today's attack on critical energy infrastructure, Deere said. "Violent actions against civilian areas and infrastructure vital to the global economy only deepen conflict and mistrust."
The US government "remains committed to ensuring global oil markets are stable and well supplied," the spokesman said.
James Rogers, a security expert with expertise in drones and a visiting research fellow at the Department of International Security Studies at Yale University, said the United Nations has been investigating the drones, which have a range of 1,450 kilometers (900 miles). He said the range means that Houthi forces can target sites in Saudi Arabia from their stronghold in Yemen.
"It is quite an impressive, yet worrying, technological feat," he said. "Long-range precision strikes are not easy to achieve and to cause the substantial fires in Abqaiq and Khurais highlights that this drone has a large explosive yield."
The United Nations special envoy to Yemen, Martin Griffiths, said he's quite concerned over the attacks.
"The recent military escalation is extremely worrying," Griffiths said in a statement, adding that such attacks "pose a serious threat to regional security."
The UK Foreign Office called the attacks "completely unacceptable."
"Threatening civilian areas and commercial infrastructure is dangerous, provocative and completely unacceptable," the office said in a statement. "This escalation also undermines UN-led efforts to end the conflict in Yemen."
CNN's John Defterios contributed to this report.
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