The five permanent members of the UN Security Council have agreed on a draft resolution giving Iran until 31 August to suspend uranium enrichment.
Iran says its nuclear programme is entirely peaceful
The draft was circulated to all 15 members of the Council and could be voted on as early as Monday.
The US envoy to the UN, John Bolton, said the Council would consider sanctions if Iran missed the deadline.
But Russian envoy Vitaly Churkin stressed that the draft resolution did not contain the threat of sanctions.
Mr Churkin said it would encourage Iran to resume dialogue.
The draft does not mention sanctions but is a clear signal to Iran that it is facing its last best chance to gain international approval for its nuclear ambitions, the BBC's Daniel Lak at the UN says.
Iran insists its nuclear programme is entirely peaceful and is designed to meet its energy needs only, but the US and other Western nations fear it is trying to develop nuclear weapons.
The draft resolution was negotiated over the past two weeks by the five permanent Council members - Britain, China, France, Russia, the United States - and also Germany.
It follows a 12 July agreement to refer Iran to the UN Security Council for failing to respond to a package of incentives to suspend enrichment.
The resolution urges Tehran to "suspend all enrichment-related and reprocessing activities, including research and development", as demanded by the International Atomic Energy Agency (IAEA).
It says the Council would consider adopting "appropriate measures" under Chapter 7 of the UN Charter if Iran does not comply with the deadline.
Mr Bolton said that the draft imposed a "mandatory requirement" on Iran.
If Tehran did not comply, Mr Bolton said, "we will move to sanctions in the Security Council".
He said Iran faced the risk of "increasing international isolation, economic and political pressure".
"I think this resolution will put the ball back in Iran's court."
Mr Churkin emphasised however that the text did not threaten sanctions, and that further measures would be discussed if Iran failed to comply.
"To describe this resolution as a sanctions resolution is clearly misleading everybody about the purpose and the content of this resolution," he said.
"We want to make sure that the package is seen in its entirety."