Iran, under pressure from major powers to build trust over its controversial nuclear programme, says it is now ready to give up enriching uranium to 20 percent to alleviate the international community's concerns.

Iranian President Mahmoud Ahmadinejad on Tuesday reiterated Tehran's readiness to "immediately" stop production of 20 percent enriched uranium provided world powers give it the nuclear plates needed to fuel its Tehran research reactor.

Ahmadinejad, speaking in an interview on Iranian state television, was repeating to his nation what he had said September in several interviews to the US press on the sidelines of a UN General Assembly in New York.

"Production of 20 percent (enriched) fuel is not economical. It is expensive and there is no (export) market that would justify continuing to operate the plant," Ahmadinejad said.

He insisted that repeating Iran's readiness to give up enrichment to 20 percent was "a way to dismantle the pretext (used by world powers) ... who say 20 percent means one step closer to the bomb."

The offer marked a shift by Iranian officials, who previously maintained that Tehran would never give up fuel production for its research reactor which it says it needs for cancer treatment.

Iran started enriching uranium to 20 percent level in February 2010, after failed negotiations over a fuel swap which would have seen Iran shipping out 3.5 percent enriched uranium in exchange for 20 percent fuel from Russia and France.

The UN Security Council has slapped four rounds of sanctions on Iran to get it to suspend uranium enrichment, a process which can produce fuel for a reactor but which can also be used, at a high enrichment level above 90 percent, in a nuclear weapon.

Ahmadinejad has been vague about the conditions under which Iran would abandon enrichment to 20 percent, and he did not specify whether it may buy the needed fuel directly from the major powers or get it in exchange for its 3.5 percent enriched uranium.

He, however, said that Iran would continue to produce low-enriched uranium for future nuclear power plants.

Ahmadinejad's remarks have been met with scepticism by members of the Western diplomatic community in Tehran.

They point out that Tehran has this year already stepped up its 20-percent enrichment drive by installing new and faster centrifuges and is transferring the activity to the underground plant of Fordo, 150 kilometres (95 miles) south of Tehran, where it will be better protected against possible air strikes compared to the current site of Natanz in central Iran.

Iran's arch-foe the United States immediately denounced Ahmadinejad's comments as "empty promises." It said that if Iran has "a serious proposal" to put forward, it should make it to the IAEA, the UN nuclear energy watchdog.

The shift in the Iranian tone comes as the West, however, continues to ratchet up economic sanctions on Tehran, cutting off finance, investment and the import of Western technology to the all-important oil and gas sector.

Difficulties also appear to have delayed the production of fuel plates needed for Tehran's research reactor, originally scheduled for September.

Russia, traditionally more lenient towards Iran and the provider of the Islamic republic's first nuclear power plant, has called on Tehran to show "more transparency" about its nuclear programme.

It has proposed a "step-by-step" process to revive talks between Iran and major powers and suggested Tehran give up building new centrifuges as an initial step.