Inspectors from the UN's nuclear watchdog have arrived at Iran's main uranium conversion plant which is set to restart operations.
Seals had been placed at the Isfahan plant during the suspension
The team are due to install surveillance equipment and oversee the removal of seals at the plant.
Iran maintains its right to carry out nuclear activity for peaceful purposes, and has rejected the latest European proposals for its nuclear programme.
Meanwhile, Iran says its ex-state media chief is to lead nuclear negotiations.
The ultra-conservative Ali Larijani will take over from Hassan Rohani as Iran's chief nuclear negotiator, the official IRNA news agency reports.
Nuclear work at the Isfahan plant had been suspended since November 2004.
"The agency technicians have arrived at the uranium conversion facility to install surveillance cameras," an official who did not wish to be identified told Reuters news agency.
"Later, the seals will be removed," he added, without specifying a time.
The International Atomic Energy Agency (IAEA) has said it will take until mid-week to install the surveillance equipment.
Iran had planned to reopen the plant a week ago, but the IAEA said it needed more time to install its cameras there.
The BBC's Frances Harrison says that even though Tehran is defying Europe by going ahead with its nuclear work, it wants everything to be done under international supervision.
Meanwhile, the EU has called an emergency meeting of the IAEA's board, which is scheduled to take place on Tuesday.
The IAEA is expected to urge Iran not to resume work at Isfahan.
French Foreign Minister Philippe Douste-Blazy has warned the issue could go before the UN Security Council if Iran did not abandon its plans to unseal the plant.
Iranian officials though said that possibility does not bother them, and that they have the rights to take this step under international treaties.
The European Union - through Britain, Germany and France - has been trying to find a compromise solution over Iran's nuclear plans for two years.
But correspondents say the latest proposals fail to contain any fundamental new concessions.
The EU plan - supported by the US - reportedly offers recognition of Iran's right to produce nuclear power for civilian purposes, as well improved trade relations with the EU and guarantees of alternative nuclear fuel sources from Europe and Russia.
In return, the Europeans reportedly insist Tehran should permanently give up nuclear enrichment and construction of a heavy-water reactor, which could be used to make a bomb.
Last week, Tehran said work at the uranium conversion plant near Isfahan would start again on Wednesday and cited lack of progress in talks with the Europeans.