REUTERS - Iran will fulfill its commitments under the July nuclear agreement with major powers in time to have sanctions, that have crippled its economy, lifted by the end of the year, its atomic energy chief said on Thursday.

President Hassan Rohani reaffirmed last week he expected sanctions to be lifted by year-end, paving the way for the return of the biggest economy to the global trading and financial system since the end of the Soviet Union in 1991.

Iran has begun work to remove uranium enrichment centrifuges as part of the landmark agreement, Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran announced this week.

"Whatever the president says we will accomplish," Salehi told Reuters after a speech on Iran's nuclear future to diplomats and energy executives in Tokyo.

Under the July 14 accord with the United States and other countries, Iran must dismantle large parts of its disputed nuclear program before international sanctions, imposed over suspicion it had bomb-making purposes, can be lifted.

Most analysts expect this process, which began on October 18th, to take at least four to six months, but Rohani has repeatedly said he expects sanctions to be lifted in December.

"As far as the dismantling of the centrifuges ... we anticipate no particular technical problems, because we have gone through this routine a number of times and our experts and engineers are well rehearsed," Salehi said during the speech.

On the issue of the Arak heavy water reactor, which must be reconfigured so it cannot produce weapons-grade plutonium, Salehi said Iran was waiting for an official document from the six powers.

"Until that document is produced we certainly will not take any measures vis-à-vis the Arak heavy water research reactor," Salehi said.