The head of the International Atomic Energy Agency said Thursday his probe of allegations that Iran tried to make nuclear arms is at "a dead end" because the government is not cooperating.

Mohamed ElBaradei warned that confidence in Tehran had shrunk in the wake of its belated revelation of a previously secret nuclear facility.

ElBaradei also criticized Tehran for not accepting an internationally endorsed plan meant to delay its ability to make such weapons.

China reportedly is considering support for the resolution that would commit Tehran to ship out 70 percent of its enriched uranium for processing into fuel rods for its research reactor.

By removing most of the material that Iran could use to make a nuclear weapon, the resolution would ease international fears and also render it impossible for Iran to produce nuclear arms for at least a year.

Vice Foreign Minister He Yafei told Reuters that the plan, to be voted on by the 35-nation IAEA governing board later this week, was meant as a signal "that Iran should indeed respond to the IAEA proposal as soon as possible."

"At the same time, we hope that this issue can be resolved through consultation," he added.

But ElBaradei said talks with Iran are at a standstill.

"There has been no movement on remaining issues of concern which need to be clarified for the agency to verify the exclusively peaceful nature of Iran's nuclear program," he told the IAEA's 35-nation board of governors at its opening session.

"We have effectively reached a dead end, unless Iran engages fully with us."

Those "issues of concern" include intelligence and other information indicating that Tehran has experimented with nuclear weapons programs — among them, missile-delivery systems and tests of explosives that could serve as nuclear-bomb detonators.

ElBaradei has been trying to persuade Tehran to freeze uranium enrichment and meet other international demands meant to ease fears of its nuclear aims since revelations of a secret Iranian nuclear program surfaced eight years ago.

But Iran started stonewalling the agency over a year ago saying the allegations were false and there was nothing to investigate.

ElBaradei has emphasized the need for talks instead of threats in engaging Iran and criticized the U.S. for invading Iraq on the pretext that Saddam Hussein had a nuclear weapons program.

But ElBaradei's comments Thursday left little doubt that — just days before his departure — he was most unhappy with Iran. Especially it's reluctance to approve the resolution, which already has the support of five other world powers.

"I am disappointed that Iran has not so far agreed to the original proposal" involving removal of most of Iran's enriched stockpile, ElBaradei told the meeting.

Tehran's approval "would greatly help to alleviate the concerns relating to Iran's nuclear program," he added.

Iran says it is enriching for peaceful energy purposes and continues to do so despite U.N. sanctions meant to make it enrichment. The defiant country has also assembled an enriched stockpile that could arm two nuclear warheads.

Initially, Tehran appeared to favor the plan. But in recent weeks it has indicated a refusal to ship out most of its enriched stockpile.

The other countries endorsing the plan — the U.S., Russia, Britain, France and Germany — planned to mount a new challenge to Tehran in the form of a resolution at the board meeting criticizing it for ignoring U.N. Security Council and IAEA board demands and continuing to build its enrichment program — sometimes clandestinely.

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 project.

Iran said it did not violate IAEA statues, but ElBaradei said Tehran was "outside the law" in not telling his agency about the facility much earlier. On Thursday, he said the incident reduced "confidence in the absence of other nuclear facilities under construction in Iran which have not been declared to the agency."

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.

The Associated Press contributed to this report.