The International Atomic Energy Agency has passed a resolution calling on Iran to suspend uranium enrichment - a key process used to make nuclear weapons.
The UN wants Iran to end enrichment activities
It also asked Iran to grant access to IAEA inspectors and provide them with any further information needed.
The resolution imposes an indirect deadline of 25 November, when the IAEA board of governors is next set to meet.
Iran's delegate said a decision "about a range of suspension" would be taken in Tehran in the next few days.
The resolution follows several days of intense negotiations in Vienna.
The talks centred on allegations, voiced in the US and elsewhere, that Iran's nuclear enrichment programme could be used to make material for nuclear weapons.
Chief US delegate Jackie Sanders said the resolution "sends an unmistakeable signal to Iran that continuing its nuclear weapons programme will bring it inevitably before the Security Council".
"The time for decisive action is approaching," she said.
"With every passing week, Iran moves that much closer to reaching the point where neither we, nor any other international body, will be able to prevent it from achieving nuclear weapons capacity."
The US, Britain, France and Germany submitted the resolution, which expresses serious concern that Tehran has not heeded previous calls to suspend its uranium enrichment activities.
The resolution also asks the head of the IAEA, Mohamed ElBaradei, to submit reports reviewing Iran's co-operation with the agency.
But other board members have expressed reservations over the resolution and Iran insists on its right to enrich uranium for peaceful purposes.
The UN nuclear watchdog keeps open the option of further steps if Iran fails to comply with IAEA demands that could include taking Tehran before the UN Security Council.
Iran says its nuclear programme is peaceful and not a matter for the UN.
Its chief delegate at the Vienna talks, Hossein Mousavian, said Tehran welcomed the decision of the IAEA board of governors "to come to [a] final conclusion in November meeting".
"We will decide in the coming days about a range of suspension which the board today have asked Iran for a full suspension," Mr Mousavian said.