Iran has pushed back by a few days its plans to restart controversial nuclear activities, says Tehran's chief negotiator Hassan Rohani.
Tehran initially said it hoped to resume uranium conversion on Wednesday - the day ultra-conservative President Mahmoud Ahmadinejad was confirmed.
But it has delayed to allow the UN to install necessary equipment to oversee activities at Iran's Isfahan plant.
Tehran's recent move has threatened to derail talks with the European Union.
"We hope to restart work by the beginning of next week when preparations are complete," Mr Rohani told state television. Iran's week begins on Saturday.
And he rejected as "unacceptable" a warning by Britain, France and Germany - the three EU countries negotiating with Iran - that they may refer Iran to the UN Security Council.
"The three European ministers have said that if we restart, this would mean the end of negotiations. This is a threat, this is unacceptable," he said.
"There is no judicial or political logic to send the issue to the Security Council. This would mean that the Europeans have given in to US pressure and they must assume the consequences.
"Once the Isfahan plant restarts, we want to continue the negotiations with the Europeans," he added.
Iranian officials have been adamant about this dramatic change of policy, even though they know it will mean an end to two years of nuclear talks with Europe, says the BBC's Frances Harrison in Tehran.
Foreign policy crisis
Earlier, Ali Aghamohammadi of Iran's Supreme National Security Council told reporters: "We hope that today we will be able to restart the activities."
He said a request by the UN's nuclear watchdog, the IAEA, to wait a week to allow surveillance systems to be installed was "not unacceptable".
Mr Ahmadinejad has been confirmed as president
On Wednesday, the former mayor of Tehran, Mahmoud Ahmadinejad, was formally endorsed as president. He will take the oath of office in parliament on Saturday.
One of his first challenges will be to deal with what looks set to be a major foreign policy crisis over Tehran's nuclear plans, say correspondents.
Iran suspended uranium conversion and enrichment activities in November, but insisted the move was only temporary.
Enriched uranium can be used to fuel nuclear power stations, but can also provide material for nuclear weapons.
The move has prompted warnings from the international community urging Iran to rethink and warning of serious consequences, including referral to the UN Security Council.
Iran says it has a right to a peaceful nuclear programme. The US believes it is seeking nuclear weapons.
EU negotiators are due to submit shortly full proposals for nuclear, economic and political co-operation with Iran, provided it ends all nuclear activities.