A ballistic missile fired by Yemeni rebels at the Saudi capital of Riyadh came from Iran, confirms a spokesman for the U.S. Air Force. The civil war in Yemen has been intensifying recently and this latest development could push it into a broader conflict that could potentially ensnare American military forces.

Lt. Gen Jeffrey L. Harrigan oversees Air Forces Central Command in Qatar. He made the revelation in Dubai at a news conference. He said they are looking into how the missile was smuggled into the country. The Saudis control Yemen’s airspace, borders, and ports.

At this time they are unclear what kind of missile it was but are sure it came from Iran:

“After the Nov. 4 strike near Riyadh, Saudi Arabia’s Foreign Ministry said investigators examining the remains of the rocket found evidence proving ‘the role of Iranian regime in manufacturing them.’ It did not elaborate, though it also mentioned it found similar evidence after a July 22 missile launch. French President Emmanuel Macron similarly this week described the missile as “obviously” Iranian.

“Nikki Haley, the U.S. ambassador to the United Nations, said in a statement Tuesday that the July launch involved an Iranian Qiam-1, a liquid-fueled, short-range Scud missile variant. Iran used a Qiam-1 in combat for the first time in June when it targeted Islamic State group militants in Syria over twin militant attacks in Tehran.”

Saudi Arabia often accuses Iran of giving weapons to Shiite rebels in Yemen. Tehran has denied doing so. The Yemen civil war is a proxy war, in many ways, between the Sunni led Saudi Arabia and the Shiite led Iran.

But things could be reaching a dangerous crescendo. Saudi Arabia claims this is an “act of war” and vows retaliation:

“The attempted missile strike was ‘a blatant act of military aggression by the Iranian regime and could rise to be considered as an act of war,’ the Saudi Press Agency said in a statement.

“Saudi Arabia ‘reserves its right to respond to Iran in the appropriate time and manner, in accordance with international law and based on the right of self-defense,’ the statement continued.”