US President Donald Trump has urged other members of the UN Security Council to work with America to ensure Iran never acquires a nuclear bomb.
Chairing a session on weapons of mass destruction, he defended re-imposing sanctions on Iran because of its "malign conduct".
He accused Iran and Russia of "enabling" "butchery" in Syria.
However he also thanked all three countries for pulling back from an offensive against rebels in Idlib.
The US president is in the Security Council chair on Wednesday because America currently holds the rotating presidency.
US-Iran relations have been sour for decades and Mr Trump has rarely lost an opportunity to criticise Tehran.
"I ask all members of the Security Council to work with the United States to ensure the Iranian regime changes its behaviour and never acquires a nuclear bomb," he said.