The US permanent representative to the UN, Nikki Haley, has accused Iran of supplying Yemen's rebel Houthi movement with missiles to attack Saudi Arabia.
She showed reporters the remnants of a ballistic missile that came close to hitting Riyadh's airport last month.
It "might as well have had 'Made in Iran' stickers" on it, she said, adding that Iran was violating UN resolutions.
Iran denies arming the Houthi movement, which has fought a Saudi-led coalition backing Yemen's government since 2015.
Iran said the claims were "irresponsible, provocative and destructive".
But Mrs Haley said several technical details - the absence of stabiliser fins and a series of valves on the side - marked the missile as Iranian-made.
She added that it had the potential to kill hundreds of civilians, and that this highlighted the "undeniable fact that the Iranian regime's behaviour is growing worse".
"We must speak with one voice in exposing the regime for what it is - a threat to the peace and security of the whole world."
Mrs Haley said she was taking the unusual step of declassifying the information "in the spirit of transparency and international co-operation".
"International peace and security depends on us working together against Iranian aggression," she said.
Mrs Haley also said the US would build an international coalition to push back against Iran, which she said was "fanning the flames" of conflict in the Middle East. She said this would include diplomatic measures.
Iran's mission to the UN said the evidence was "fabricated" to serve a US agenda.
"These accusations seek also to cover up for the Saudi war crimes in Yemen, with US complicity," a statement by the mission's spokesman read.
A report by UN Secretary General António Guterres concludes that debris from the missile fired at Riyadh, and another shot down near Mecca in July, points to a common origin, according to AFP news agency.
But the report does not firmly conclude that the missiles came from Iran.
Iran's foreign ministry has previously said November's missile launch was "an independent action" by the Houthis in response to Saudi-led coalition "aggression".