VIENNA, Austria - The head of the International Atomic Energy Agency has urged Iran to endorse a plan that would strip it of most of its enriched uranium, saying Tehran could not defuse fears about its nuclear program with proposals that included keeping the material.

IAEA chief Mohamed ElBaradei's comments Wednesday were his firmest public rejection to date of Iranian attempts to modify a proposal that would involve shipping out around 70 per cent of its enriched stockpile and returning it in the form of fuel rods for its Tehran research reactor.

While Iran has offered several counterproposals - buying the rods from abroad or exchanging its enriched uranium in small batches - all have in common Tehran's rejection of exporting most of its enriched uranium.

Iran now has enough enriched uranium for up to two nuclear weapons. If stripped of 70 per cent of that material, its ability to make such arms would be delayed for at least a year.

Tehran insists it wants to enrich only to power an envisaged nuclear reactor network. But fears that it could instead turn to making fissile highly enriched uranium for warheads have resulted in U.N. Security Council demands that it freeze enrichment - and three sets of U.N. sanctions, shrugged off by Tehran.

"You need the material (out) from Iran to defuse the crisis and open the space for negotiations," ElBaradei told reporters. "Keeping the material in Iran will not lead to that."

His comments dovetailed with the view of six powers endorsing the plan - the U.S., Russia, China, Britain, France and Germany.

Those same nations planned to mount a new challenge to Tehran this week in the form of a resolution to a 35-nation IAEA board meeting criticizing it for ignoring U.N. Security Council and IAEA board demands and continuing to build its enrichment program - sometimes clandestinely.

Two diplomats who demanded anonymity because their information was confidential said that by Wednesday, the eve of the board's opening session, close to two-thirds of board members had expressed support for the resolution in private meetings.

Impatience with Iran has been fueled by Tehran's September revelation that it had secretly been building a new enrichment facility. In a possible pre-emptive move, Iran notified the IAEA in a confidential letter only days before the leaders of the U.S. Britain and France went public with the clandestine project.

Iran says it did not violate IAEA statues by waiting with its notification. But ElBaradei has said Tehran was "outside the law" in not telling his agency about the facility much earlier. On Wednesday, he said that Iran's secrecy on the facility reduced "overall confidence" that Tehran is telling the truth when it asserts it is not interested in nuclear arms.

A perusal of IAEA records shows that Tehran's chief envoy to the IAEA, Ali Asghar Soltanieh, told the agency's board last year that his country "has repeatedly declared that there is no undeclared nuclear material and activity in Iran" - at the time when construction of the secret nuclear facility was in full force.

A copy of the six-power resolution prepared for the closed meeting and obtained by The Associated Press noted that Iran's delay in reporting the new facility "does not contribute to the building of confidence" in Tehran's nuclear aims.

The facility "gives rise to questions about whether there are any other nuclear facilities under construction in Iran which have not been declared to the agency," the document says.

The resolution is significant because it groups Russia and China with the four Western powers in unified criticism of Iran's nuclear program. Russia and China have acted as a drag on Western calls for tougher action against Iran.

While the board passed an IAEA resolution critical of Iran in 2006 with the support of all six world powers, subsequent attempts by the West to get backing from all 35 board nations foundered on resistance from Russia and China.

Those two nations have also resisted U.S. and European calls for tougher U.N. sanctions against Iran for refusing to freeze its enrichment program.

Such unified action in Vienna could signal that both Russia and China may be more amenable to a fourth set of Security Council sanctions on Iran than they have been in past years.

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