VIENNA, Austria - The chief UN nuclear inspector said Monday his agency cannot be sure if Iran's nuclear activities are peaceful or a cover for a weapons program until the country co-operates with his experts.

Mohamed ElBaradei, the head of the International Atomic Energy Agency, spoke as board member nations of the IAEA gathered for a session on approving the suspension of dozens of technical aid programs to Iran as part of Security Council sanctions meant to punish the country for its nuclear defiance.

Although the issue is not expected to come up until Tuesday at the earliest, the focus of the IAEA's 35-nation board meeting will be on Iran's refusal to suspend uranium enrichment activities and linked problems.

The board also will be reviewing another key nuclear issue -- North Korea's apparent willingness to ultimately dismantle its nuclear arms-making capabilities based on the Feb. 13 agreement when it pledged to take initial steps to dismantle its atomic program in return for aid.

Ali Ashgar Soltanieh, Iran's chief delegate to the IAEA, again said his country would "never give up its inalienable right" to develop enrichment -- which Iran says it wants to develop to generate power but which also can produce the fissile material for nuclear warheads.

ElBaradei told reporters that Iran appeared to have paused in developing its enrichment program amid Security Council deliberations on sharpening existing sanctions imposed because Tehran refuses freeze enrichment activities.

"I do not believe that the number of centrifuges has increased, nor do I believe that nuclear material has been introduced to the centrifuges at Natanz," he said, referring to the machines used to enrich uranium. "The situation today is still very much R&D activities."

In opening remarks to the gathering, ElBaradei said that despite four years of probing Iran's nuclear activities, the IAEA remains "unable to provide the required assurance about the peaceful nature of Iran's nuclear program."

International concern would not subside until the country takes "the long overdue decision" to co-operate with the IAEA, he said.

"Quite a few uncertainties still remain about experiments, procurements and other (nuclear) activities," he said, alluding to a constant theme in IAEA reports over the past years -- refusal by Iran to meet agency requests for clarification about aspects of its program with possible weapons applications.

Most recently, diplomats familiar with the agency's Iran file said before the closed meeting that the country continues to refuse IAEA requests to put up cameras that would give agency monitors a full view of in its underground hall at Natanz, which Iran says will ultimately house 54,000 enriching centrifuges -- enough to produce dozens of nuclear weapons a year.

One of the diplomats, who demanded anonymity because he was not allowed to discuss the confidential file with the media, said Iran continues to assemble individual centrifuges in the hall.

The Islamic republic insists it has a right to enrichment to generate nuclear energy. But growing fears about the program's other application -- creating the fissile material for nuclear warheads -- led to sanctions last year from the UN Security Council.

Iran's decision in late January to bar 38 inspectors from entering the country also was straining relations with the IAEA, said another diplomat. Iran accused one senior expert of "spying for his home country" in 2006 by using wiretapping equipment to collect information outside the purview of nuclear inspections, the diplomat said.

Up for review will be a Feb. 22 report from ElBaradei finding that Iran has expanded enrichment, violating the Security Council ultimatum and leading to a new round of debate on widening sanctions.

As well, 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 had faced such punishment in the past.

ElBaradei, in an internal report circulated to board members last month, had called for full or partial suspension of 18 projects that he deemed could be misused to create nuclear weapons.

The board in the past has often split on what action to take against Iran. The United States, its key allies and most European nations have usually been opposed by nonaligned board members who were against harsh punishment.

But the diplomats said that even nations normally backing Iran -- including key U.S. critics such as Cuba and Venezuela -- would likely agree to the suspensions because they were backed by the UN Security Council.

ElBaradei plans to go to Pyongyang March 13 as part of the six-nation agreement under which North Korea would allow a return of his agency's experts under its commitment to eventually scrap its nuclear program in exchange for economic aid and security assurances.

North Korea kicked IAEA monitors out in late 2002, at the beginning of the current nuclear standoff, withdrawing from the Nuclear Nonproliferation Treaty and reactivating its mothballed nuclear program, which led to its first atomic weapons test in October.

The North's main nuclear envoy, Kim Kye Gwan, told his South Korean counterpart, Chun Yung-woo, last week that the communist nation's pledge to disable its nuclear weapons was irreversible, a South Korean Foreign Ministry official said Monday. The official spoke on condition of anonymity because of department policy.

North Korea's main nuclear envoy, Kim Kye Gwan, prepared to meet with his U.S. counterpart, Christopher Hill, later Monday in New York on normalizing relations, one of the agreements reached under the Feb. 13 nuclear accord. Similar talks between the North and Japan are also planned this week in Hanoi, Vietnam.

Also at the Vienna meeting, a letter from 17 Arab nations plus Palestinian authorities was submitted to the IAEA and made available to The Associated Press that called for Israel to be put under agency inspections, asserting that Prime Minister Ehud Olmert last year acknowledged that his country had nuclear weapons -- something Olmert has denied doing.