Russia and China have joined Western powers in telling Iran its "consistent failure" to comply with UN resolutions "deepened concerns" about possible military dimensions to its nuclear programme.

The United States, Germany, France, Britain, Russia and China issued the statement on Thursday a day after Iran said it would triple production of high-grade uranium and shift it to an underground bunker which would be protected from possible US or Israeli air strikes.

Russia and China have in the past been less inclined to try to isolate Iran over its uranium enrichment which Tehran says is solely to generate electricity.

Western nations suspect the Islamic Republic wants to enrich uranium to a higher level so as to be able to make nuclear weapons if it chooses.

"We call on Iran to cooperate fully with the agency," said a joint statement by the six powers at a closed-door session of the International Atomic Energy Agency (IAEA) in Vienna.

"Outstanding issues need to be resolved in order to exclude the existence of possible military dimensions to Iran's nuclear programme." It did not mention Iran's plan to expand enrichment.

Washington's envoy said separately that Iran's plan to speed up enrichment with a more advanced model of centrifuge machines in a mountain bunker showed its "brazen" and deepening defiance of international demands to curb the activity.

Iran's refusal to halt enrichment has led to four rounds of UN sanctions on the major oil producer, as well tighter US and European Union restrictions on top.

World powers have also offered Tehran trade incentives to give up enrichment, but President Mahmoud Ahmadinejad said on Tuesday no inducement could persuade Iran to stop. US President Barack Obama said further sanctions were likely.

Iran's IAEA representative hit back at a tense board meeting of the UN nuclear watchdog, vowing to resist Western pressure.

Ambassador Ali Asghar Soltanieh launched a verbal attack on IAEA Director-General Yukiya Amano and accused him of bias.

The Japanese IAEA chief has taken a blunter approach to Iran than his predecessor Mohamed ElBaradei, saying in his first report on the country early last year that he feared it may be working to develop a nuclear-armed missile.

"He is not doing his job. Instead, with his reports, he is paving the way for more confrontation between member states," Soltanieh told reporters.

But the six powers united to throw their weight behind Amano, who last month asked Tehran to provide access to sites and officials to help answer the agency's queries. Iran rebuffed the request.

The US and Israel, Iran's arch-adversaries, have not ruled out military action to knock out Iranian nuclear assets if diplomacy fails to resolve the eight-year-old dispute.

Iran says it is refining uranium to the 3.5 percent level for electricity production and to 20 percent purity for medical applications.

But its decision last year to raise the level of enrichment beyond that needed for power plant fuel to 20 per cent increased disquiet in the West because this would bring Iran significantly closer to the 90 per cent threshold suitable for bomb fuel.-Reuters