The United Nations is “unable to independently corroborate” that missiles and drones used in the attacks on Saudi oil facilities in September “are of Iranian origin,” Secretary-General Antonio Guterres told the Security Council on Tuesday.

The United States, European powers and Saudi Arabia have blamed the September 14 attack on Iran. Yemen’s Houthi rebels had claimed responsibility for the attacks, and Iran, which the group is closely aligned to, denied any involvement.

Guterres said the UN examined the debris of weapons used in attacks on a Saudi oil facility in Afif in May, on the Abha international airport in June and August and on the Saudi Aramco oil facilities in Khurais and Abqaiq in September.

“At this time, it is unable to independently corroborate that the cruise missiles and unmanned aerial vehicles used in these attacks are of Iranian origin,” he wrote in the report, seen by the Reuters news agency.

The attacks that targeted the Abqaiq and Khurais oil plants caused a spike in oil prices and shut down more than 5 percent of global oil supply. Saudi Arabia said on October 3 that it had fully restored oil output.

UN experts monitoring Security Council sanctions on Iran and Yemen travelled to Saudi Arabia days after the September 14 attack.

The report noted that Yemen’s Houthis “have not shown to be in possession, nor been assessed to be in possession” of the type of drones used in the attacks on the Aramco facilities.

Guterres reports twice a year to the Security Council on the implementation of an arms embargo on Iran and other restrictions that remained in place after Tehran agreed to a nuclear deal with world powers in 2015.

The Council is due to discuss his report next week.

A separate independent panel also reports twice a year to the Security Council on the implementation of sanctions related to the conflict in Yemen, which were imposed in 2014 and 2015. That report is due next month.

Saudi Minister of State for Foreign Affairs Adel al-Jubeir told reporters in New York in September that his country had consulted with its allies on what steps to take after the attacks.

“The United Nations sent people to be part of the investigation, other countries have sent experts to be part of the investigation,” he said then. “When the team that’s investigating has concluded its investigations we will make the announcements publicly.”