The head of a United Nations' nuclear watchdog, the International Atomic Energy Agency warns that it will take some time before work can begin on verifying whether Iran is complying with a deal on its nuclear programme.
Iran agreed in late November to curb some nuclear activities for six months in return for sanctions relief, the BBC reports.
The deal requires increased UN inspections of Iran's nuclear sites including the Arak heavy water plant.
But the head of the UN nuclear agency, Yukiya Amano, says he can not yet tell when the inspections will begin.
Under the international deal, Iran will receive some $US7 billion in sanctions relief while talks continue to find a more permanent agreement.
The BBC reports that the stipulated six-month interim period will only begin once technical issues such as inspections have been worked out.
The accord reached with the US, UK, France, Russia, China and Germany, has been generally welcomed.
But Israeli Prime Minister Benjamin Netanyahu has called it a "historic mistake" and some US senators say it is too soft.
The West has long suspected that Iran's uranium enrichment programme is geared towards making a weapon, but Tehran insists it only wants nuclear energy.
The Vienna-based International Atomic Energy Agency (IAEA) is expected to play a key role in verifying Iran's compliance, including significantly expanding its inspections of nuclear sites.
Mr Amano, speaking at IAEA headquarters, told reporters: "It will take time because it is a quite complicated task and we would like to properly prepare and do the job properly. I cannot tell when we will be ready."
He indicated that the agency would be seeking special funding for the operation, which he said would "requires a significant amount of money and manpower".