(Bloomberg) -- Cruise missiles that slammed into a Saudi oil complex last year likely came from Iran, the United Nations concluded in a confidential report, supporting U.S. allegations that the Tehran government was behind the attack, which sent tremors through global energy markets and shook the kingdom.

The UN found that the weapons used in that attack and others were of “Iranian origin,” Secretary-General Antonio Guterres said in the semi-annual report sent Thursday to the Security Council that was seen by Bloomberg. Guterres also pointed out that “these items may have been transferred in a manner inconsistent with” UN resolutions.

Missiles and a swarm of drones set off fires at the crude processing plant in Afif in May 2019, causing extensive damage. The Houthi rebels in Yemen, who have been battling a Saudi-led coalition in that country’s civil war, claimed responsibility. But once the planning and military sophistication that went into the assault became apparent, suspicion quickly shifted to the Iranians, who have been supporting the Houthis.

Investigators analyzed the debris of the cruise missiles and drones in that attack and attacks on the Abha International Airport in southwestern Saudi Arabia that June and August; and in Abqaiq and Khurays that September.

The investigators concluded that the delta-wing drones deployed in the attacks were of Iranian origin. The engines on those aircraft showed similarities to an Iranian engine designated as Shahed 783, presented by Iran in a military exhibition in May 2014, the UN said.

The findings come as the Trump administration seeks to renew a UN arms embargo on Iran that’s set to expire this year as part of the 2015 nuclear deal, from which President Donald Trump has withdrawn.

Russia and China have already said they won’t support it. Officials including Secretary of State Michael Pompeo are warning the U.S. could reimpose all UN sanctions against Tehran if the embargo were allowed to expire.

The attacks raised questions about Saudi Arabia’s role as an anchor of stability in global energy markets and exposed glaring vulnerabilities in Saudi Arabia’s defense capabilities despite having spent hundreds of billions of dollars on weaponry.

Iran and Saudi Arabia’s struggle for dominance in the Persian Gulf and beyond has often drawn in the U.S. Trump, after rejecting the nuclear accord negotiated by the Obama administration, has cultivated closer relations with the Saudi rulers at the same time as he has imposed harsher sanctions on Iran.

Tensions flared most recently in January after the U.S. killed a top Iranian general, Qassem Soleimani, who ran the Quds Force and oversaw the Islamic Republic’s ties to armed groups in the Middle East.

Earlier this week, Iran sentenced a man to death on charges of providing the CIA and Israel’s Mossad with intelligence that led to the killing of Soleimani in a drone strike in Baghdad, according to the country’s judiciary.

©2020 Bloomberg L.P.