"Nothing's changed. The IAEA (International Atomic Energy Agency) team still needs until the middle of this week to get the inspection system set up and in place before the facility could be restarted," spokeswoman for the UN atomic agency Melissa Fleming said in Vienna on Monday.
"We continue our request to Iran to refrain from breaking any seals until the safeguards are in place," she added.
IAEA inspectors arrived on Monday at Isfahan to install surveillance equipment and oversee the removal of seals as Tehran prepares to resume work there.
"The agency technicians have arrived at the uranium conversion facility to install surveillance cameras," a senior Iranian official said on Monday on condition of anonymity.
"Later, the seals will be removed."
The official did not specify when the Isfahan plant would restart uranium conversion work, although state television said it could take place around midday (0730 GMT) on Monday.
Such a move would lead Tehran into direct confrontation with the European Union, which has warned that Iran faces being referred to the UN Security Council for possible sanctions if it resumes work at the plant near the central Iranian city of Isfahan.
Iran has said it will resume the work, despite the risk of being hauled before the Security Council.
The crisis has escalated since President Mahmoud Ahmadinejad took office last week, with the new leader on Monday putting a new person in charge of the nuclear dossier.
New chief negotiator
Former state broadcasting head Ali Larijani, a conservative with close ties to Supreme Leader Ayat Allah Ali Khamenei, will replace Hassan Rohani as Iran's chief nuclear negotiator, a government spokesman said on Monday.
Ali Larijani will be the new chief
nuclear negotiator for Iran
The spokesman said Larijani would soon take up the post as secretary of Iran's Supreme National Security Council (SNSC).
Larijani replaces Rohani, who has managed to maintain dialogue with the West through thick and thin over the last two years, and his appointment will worry some Western negotiators.
The new SNCS secretary has likened giving up Iran's right to uranium enrichment in exchange for EU incentives to swapping "a pearl for a sweet".
Tehran denies US accusations that its nuclear programme is a front for bomb making, saying it quickly needs to develop nuclear power as an alternative energy source to meet booming electricity demand and preserve its oil and gas reserves for export.
Britain, Germany and France have called an emergency meeting of the International Atomic Energy Agency to be held on Tuesday to warn Iran not to resume work at Isfahan.