Iranian President Mahmoud Ahmadinejad has said Tehran will not yield to international pressure to stop its sensitive nuclear work.
Mr Ahmadinejad maintains Iran has a right to a nuclear programme
"Iran will not back down an inch... and will not accept being deprived of its rights," he said in a speech.
The UN had set a 31 August deadline for Tehran to suspend uranium enrichment and re-processing activities.
If Iran is found not to comply, the US wants UN powers to discuss a resolution which could impose sanctions on Iran.
The International Atomic Energy Agency (IAEA) is due to submit a report to the UN Security Council which is expected to say that Iran has not complied with the UN demand.
UN Secretary General Kofi Annan said the deadline had passed but he did not expect immediate action from the world body.
"Even though the deadline has expired I don't think the Council is going to act tomorrow," he said, on a visit to Jordan.
EU foreign policy chief Javier Solana and Iran's chief nuclear negotiator Ali Larijani are to holds talks in coming days.
The two agreed in a telephone call "to have a face-to-face meeting soon", Mr Solana's spokeswoman said, adding that a date and venue has yet to be decided.
US ambassador to the UN John Bolton has said Iran is well aware that ignoring the UN demands could trigger sanctions.
He said the five permanent members of the Security Council had repeatedly warned that failure to meet the deadline would result in them seeking sanctions.
US State Department spokesman Sean McCormack said US Under-Secretary of State Nicholas Burns and senior officials from the UK, France, Russia, China and Germany would meet in Europe early next week to discuss the issue.
What form any sanctions against Iran would take has yet to be determined.
Russia and China, which can both veto action at the Security Council, have urged patience and said they would not support severe punishments.
Iran maintains it has a right to a nuclear programme which, it says, has a purely civilian aspect.
But Western powers accuse Iran of trying covertly to develop a nuclear bomb.
A senior White House official has told the BBC the US continues to detect low level uranium enrichment by Iran.
Such activity would be in defiance of demands from both the UN and International Atomic Energy Agency (IAEA).
The official, speaking on condition of anonymity, said the Iranians were trying "to familiarise themselves with the process of enriching uranium" and the technology associated with the product.
However, the official said the end product was not of a superior enough quality to be used for nuclear weapons.
On Saturday Mr Ahmadinejad inaugurated a new phase of a heavy water reactor project in Arak.
A by product of heavy water reactors can be plutonium which can be an alternative route to a nuclear device, the other being highly enriched uranium.
Observers say the move was aimed at sending out a signal of defiance ahead of the Security Council's deadline.
After inaugurating the heavy water plant, Mr Ahmadinejad again said Iran would never abandon its nuclear programme, but that nuclear weapons were not its goal.