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.
The Trump administration has abandoned an international deal reached with Iran in 2015 which lifted sanctions in return for steps by Iran to reassure the world it was not seeking to develop nuclear weapons.
That move has exposed divisions between the US and its European allies.
The US president also criticised China at the Security Council meeting, saying it was attempting to "meddle" in the forthcoming US mid-term elections to prevent his Republican party winning.
How does Iran defend itself?
It was not attending the Security Council meeting because it is not currently a member of the body but its president did respond to scathing criticism levelled by Mr Trump in a speech to the UN General Assembly on Tuesday.
Hassan Rouhani described the US as a "bully", telling the General Assembly that dialogue should begin by ending threats and what he called "unjust sanctions".
No nation could be brought to the negotiating table by force, he argued.
"The economic war that the United States has initiated under the rubric of new sanctions not only targets the Iranian people but also entails harmful repercussions for the people of other countries, and that war has caused a disruption in the state of global trade," he said.
"What Iran says is clear: no war, no sanctions, no threats, no bullying; just acting according to the law and the fulfilment of obligations."
The remaining signatories in the nuclear deal - the UK, China, France, Germany and Russia - say they will set up a new payment system to maintain business with Iran and bypass US sanctions.
US Secretary of State Mike Pompeo condemned the idea as "one of the most counterproductive measures imaginable".
What did Trump say about Israel?
At a meeting with Israeli Prime Minister Benjamin Netanyahu before Wednesday's session, Mr Trump said he thought a two-state solution was the best option for Israel and the Palestinians.
He added that it was a "dream" for him to unveil a peace plan by the end of his first term.
The Trump administration has previously said it will support a two-state solution if both parties agree to it.
Source: bbc