The UN nuclear watchdog said Thursday it was not yet ready to verify Iran's compliance with the recent deal with world powers, as Tehran invited inspectors to the key Arak site.

"We need to study the agreement (struck in Geneva on Sunday) and we have to identify the ways in which the elements relevant to the IAEA be put into practice," International Atomic Energy Agency chief Yuyika Amano said.

"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 told reporters.

He added that the Vienna-based body would need more money to carry out the enlarged inspection role foreseen in Iran's breakthrough deal with the United States, China, Britain, France and Germany -- the P5+1.

"This requires a significant amount of money and manpower.... The IAEA's budget is very, very tight. I don't think we can cover everything from our own budget," Amano said.

Under the terms of the deal, Iran will freeze certain activities for six months in exchange for minor relief from UN and Western sanctions that have hit its economy hard.

The temporary freeze is meant to make it more difficult for Iran to develop a nuclear weapon and to build confidence while Tehran and the P5+1 hammer out a long-term accord.

This six-month period has not yet begun. The start-date will be negotiated in upcoming technical discussions that will include the IAEA. There are expectations that it will begin in January or possibly December.

Iran has pledged to limit uranium enrichment to low fissile purities. It will also lower the purity of its stockpile of medium-enriched material, which is relatively easy to convert to weapons-grade, or convert it to another form.

Iran also committed for six months "not to make further advances" at its Fordo and Natanz uranium enrichment sites and at the Arak heavy water reactor, which could provide Iran with weapons-grade plutonium once operating.

This will all have to be verified by the IAEA, meaning a considerably bigger strain on its financial and human resources.

Joseph Macmanus, US ambassador to the IAEA, said Thursday that he was "very confident that the funding will be clearly understood (by member states)... and then will be provided".

The IAEA already keeps close tabs on Iran's nuclear work, with personnel almost constantly in the country inspecting machinery and measuring stockpiles.

But under Sunday's deal this will go further, with daily IAEA visits to enrichment sites and access to centrifuge assembly sites, uranium mines, and more frequent trips to Arak -- in addition to verifying the enrichment freeze.

Iran will also have to provide information on plans for new nuclear facilities, descriptions of every building at nuclear sites and updated design information on the Arak reactor, according to the text of Sunday's agreement.

Amano also said on Thursday that Iran has invited the agency to visit the heavy water production plant at Arak on December 8 for the first inspection there since August 2011.

He said the IAEA would formally accept the invitation, saying: "I don't see a reason why we would not go there... on that date."

The IAEA already conducts regular visits of the actual reactor site at Arak, but says it has not received updated detailed design information since 2006.

Amano said the invitation was the "first concrete proposal" following the signing of a separate "joint statement on a framework for cooperation" between the IAEA and Iran signed in Tehran on November 11.