VIENNA, Austria - Iran seems to have at least temporarily halted the uranium-enrichment program at the heart of its standoff with the U.N. Security Council, the head of the International Atomic Energy Agency said yesterday.

The pause could represent an attempt to de-escalate Iran's conflict with the Security Council, which is deliberating a set of harsher sanctions on the Islamic Republic.

Iran has enriched small quantities of uranium to the low level suitable for nuclear-fuel generation. The United States and its allies fear that Iran could build nuclear weapons with larger amounts of more highly enriched uranium.

President Mahmoud Ahmadinejad had been expected to announce last month that Iran had started installing 3,000 uranium-enriching centrifuges at a facility in the desert outside the central city of Natanz, where it has about 500 centrifuges above and below ground. But the announcement never materialized, an apparent step back that IAEA chief Mohamed ElBaradei appeared to confirm yesterday.

"I do not believe that the number of centrifuges has increased, nor do I believe that [new] nuclear material has been introduced to the centrifuges at Natanz," he said.

ElBaradei, whose agency has spent more than four years probing Tehran's nuclear activities, warned that, despite the new bit of positive news, lack of Iranian cooperation left the IAEA unable to establish that Tehran's nuclear activities were purely peaceful.

Unless Tehran takes "the long-overdue decision" to cooperate, the IAEA "will have no option but to reserve its judgment about Iran's nuclear program," he told reporters.

And a senior Iranian official dashed hopes that any short-term pause could translate into Tehran's accepting a Security Council demand to freeze its enrichment activities. Ali Ashgar Soltanieh, Iran's chief IAEA delegate, said his country would "never give up its inalienable right" to enrich uranium, a refrain repeated almost daily by Iranian representatives in the standoff.

Diplomats familiar with the IAEA's Iran file said Tehran continued to refuse agency requests to install cameras that would give its monitors a full view of Iran's underground hall at Natanz. Iran has said the site will ultimately house 54,000 enriching centrifuges - enough to produce dozens of nuclear weapons a year.

Iran has produced and stored 250 tons of the gas used as the feedstock for enrichment. That would be enough to produce more than 40 nuclear bombs.

Lack of full remote monitoring means the agency cannot keep tabs on all activities at the bunker, said one diplomat, who demanded anonymity because he was not allowed to discuss the issue. Iran continues to assemble individual centrifuges without connecting them into the cascade needed to enrich uranium in the hall, he said.

Up for review as early as today will be a Feb. 22 report from ElBaradei that Tehran has set up hundreds of centrifuges.

The board was expected to approve last month's decision by ElBaradei to suspend nearly half the technical aid his agency provides to Iran. Only North Korea and Saddam Hussein's Iraq have faced such punishment in the past.