Sounding an alarm over nuclear proliferation, the head of the U.N. atomic watchdog agency said Monday that more than 40 countries have the know-how to produce nuclear weapons and that the agency is only relying on their "good intentions" to reveal all their activities.

Mohamed ElBaradei (search), head of the International Atomic Energy Agency (search), also urged Iran (search) to heed international demands to freeze technology that can be used for nuclear weapons, and cooperate with his probe of "serious concerns" about Tehran's nuclear activities.

Iran, however, has remained defiant, with one official saying his country may resume uranium enrichment at "any moment."

In a keynote address to the IAEA's general conference, ElBaradei suggested it was time to tighten world policing of nuclear activities, which until recent years had relied mostly on countries volunteering information.

Beyond the declared nuclear-arms countries, "some estimates indicate that 40 countries or more now have the know-how to produce nuclear weapons," ElBaradei said. "We are relying primarily on the continued good intentions of these countries," he said, adding that those intentions could be "subject to rapid change."

His comments appeared prompted by a series of revelations of proliferation or suspected illicit nuclear activities in the past two years.

Libya last year revealed a clandestine nuclear arms program and said it would scrap it; North Korea is threatening to activate a weapons program; Iran is being investigated for what the United States says is evidence it was trying to make nuclear arms; and South Korea recently revealed secret experiments with plutonium and enriched uranium, both possible components of weapons programs.

ElBaradei linked the need for strengthened controls to concerns about the international nuclear black market, which supplied both Iran and Libya and whose existence was revealed last year.

The "relative ease with which a multinational illicit network could be set up and operate demonstrates clearly the inadequacy" of the present controls on nuclear exports, he said.

ElBaradei's focus on Iran reflected a demand just two days earlier of the IAEA's 35-nation governing board that Iran freeze all work on uranium enrichment.

The resolution passed by the agency was its toughest yet on Tehran but didn't go as far as the United States had sought — stopping short of saying Iran will automatically be sent to the U.N. Security Council for possible sanctions if it fails to meet the demands by November.

The resolution said the board "considers it necessary" that Iran suspend all uranium enrichment and related programs. And it expressed alarm at Tehran's plans to convert more than 40 tons of raw uranium into uranium hexafluoride — the gas that when spun in centrifuges turns into enriched uranium.

Suggesting that Iran may have to answer to the U.N. Security Council (search) if it defies the demands, the resolution said the next board meeting in November "will decide whether or not further steps are appropriate" in ensuring Iran complies.

Iran's intelligence minister, Ali Yunesi, told state television, however, that his country "may resume (enrichment) any moment."

"The resolution is illegal," he said. "The Islamic Republic of Iran ... will ignore the provisions of the resolution because it is beyond the responsibilities of the IAEA."

Delivering the same message at the Vienna conference, Iranian Vice President Reza Aghazadeh said his country will "continue its nuclear activities without interruption."

U.S. Energy Secretary Spencer Abraham, in his comments to the conference, urged Iran to "cooperate fully and immediately with the IAEA's requests." And speaking for the 25-nation European Union, Dutch delegate Justus de Visser asked Tehran to "heed the content of the resolution, and in particular ... suspend fully all its enrichment-related activities."

Russia also urged Iran to comply with the resolution, saying it represented a compromise.

"The resolution gives markers on how to remove all the outstanding questions in the optimal way, through cooperation," the Russian Foreign Ministry said in a statement. "In particular, it addresses an appeal to Tehran to renew the moratorium on all enrichment work. We, too, support this appeal."

Russia is finishing work on a $800 million reactor in southern Iran, which has drawn protests from the United States and Israel. Washington fears the plant can be used to build nuclear weapons, but Iran insists it is only for peaceful energy needs. Russia says it will start shipping fuel to Iran as soon as Tehran signs a protocol on returning spent nuclear fuel to Russia for storage and reprocessing.

In his comments Monday, ElBaradei urged Iran to comply with the resolution — to "verify its past nuclear program and ... do its utmost to build the required confidence" by heeding the full suspension call.

ElBaradei also touched on North Korea, saying it "continues to pose a serious challenge" to nonproliferation

North Korea cut its ties with the agency two years ago, saying it had quit the Nonproliferation Treaty. It is now engaged in off-and-on negotiations with the United States and four other countries on aid and other concessions it seeks in return for scrapping its nuclear weapons program.

Unlike the board, the 137-nation general conference cannot set ultimatums or threaten nations with Security Council action. But it can recommend the IAEA secretariat take up matters of concern, which, in turn, can kick issues to the board.