World powers expressed "deeper concern" Thursday about the intention behind Iran's controversial atomic drive in face of its refusal to answer questions about possible military dimensions to the work.

A day after Iran announced it was expanding uranium enrichment -- the most senstive part of its nuclear programme -- Britain, China, France, Germany, Russia and the United States expressed their concerns in a joint statement to a meeting of the International Atomic Energy Agency's 35-member board of governors here.

"Iran's consistent failure to comply with its obligations under six UN Security Council resolutions and to meet the requirements of 10 IAEA board of governors resolutions has deepened concerns raised by the international community with Iran's nuclear intentions," the joint statement said.

The so-called P5+1 Group noted that IAEA chief Yukiya Amano had written to the head of Iran's Atomic Energy Organization, "reiterating concerns about the existence of possible military dimensions of Iran's nuclear programme."

It was vital that Iran clarify the issues and provide "prompt access to a range of places, individuals, and information associated with the PMD issue," they said, referring to the possible military dimensions.

Earlier, the US ambassador to the IAEA, Glyn Davies, hit out at Iran's plans to triple its uranium enrichment capacity in defiance of multiple UN sanctions.

The move was "the most recent brazen example of (Iran's) deepening non-compliance" with its international obligations, Davies said.

On Wednesday, Iran's nuclear chief Fereydoun Abbasi Davani had announced that the Islamic republic would expand its production of 20-percent enriched uranium and move the work from its main enrichment plant in Natanz to a smaller site at Fordo.

Iran has long been producing low or 3.5-percent enriched uranium (LEU) at Natanz, but started producing uranium at the higher level of purity of 20 percent in February 2010, ostensibly to make the fuel for a medical research reactor.

The UN Security Council in New York has repeatedly ordered Tehran to halt all uranium enrichment until the IAEA had verified the exclusively peaceful nature of its nuclear activities.

Uranium enrichment is the most sensitive part of the programme because it can be used to produce both the fuel for a nuclear reactor and the fissile material for an atomic warhead.

The West accuses Tehran of seeking to build a bomb under the guise of a civilian power programme, a charge which Iran strongly denies.

Iran's envoy to the IAEA, Ali Asghar Soltanieh, insisted that his country had no choice but to produce the nuclear fuel itself, because negotiations over a supply deal with the US, France and Russia had not come to fruition and enriched uranium was not commercially available to Iran via the markets.

"We are in need of nuclear fuel for the Tehran research reactor" which makes radioisotopes for medical research and the treatment of cancer, Soltanieh said.

"Hundreds of patients are struggling with cancer (and) need radioisotopes. And if we can't get the fuel from supplier countries then we have to accelerate to produce the required fuel" ourselves, he told reporters on the sidelines of the IAEA board meeting.

US ambassador Davies noted that the expansion of the enrichment capacity meant Iran would be producing more 20-percent enriched uranium than it needs for its one and only research reactor.

Furthermore, "it also represents yet another chapter in the changing Iranian narrative regarding why this underground facility was built," Davies said.

The Fordo plant was built secretly deep inside a mountain near the Shiite shrine city of Qom some 150 kilometres (95 miles) southwest of Tehran.

Revelations in 2009 about its construction infuriated the West and prompted the United Nations to strengthen sanctions against Tehran.