The United States announced Tuesday it would help developing countries track down loose radioactive materials on their soil, the kind of step the chief U.N. nuclear watchdog said was "urgently needed" to foil terrorists bent on building "dirty bombs."

The U.S. energy secretary, Spencer Abraham, said Washington would spend $3 million in the next year to help poorer governments secure high-risk radiation sources that could be used for terror weapons.

"The threat requires a determined and comprehensive international response," Abraham told the opening session of the first high-level global conference on what are technically known as radiological dispersal devices.

Though at odds over Iraq, the U.S. and Russian governments joined forces as co-sponsors to bring together hundreds of scientists and government officials for the three-day gathering, to confront a threat that hasn't materialized yet but that could plunge cities into chaos if it does.

Mohamed ElBaradei, director general of the Vienna-based International Atomic Energy Agency (IAEA), said that before the Sept. 11 terror attacks, concerns about radiation sources — the cesium, strontium and other isotopes used in medicine and industry — had focused on prevention of accidental exposure.

But now the threat of a dirty bomb — a conventional bomb packed with some radioactive material — presents "a difficult and complex challenge," he told the conference.

"It is clear that additional security measures are urgently needed."

Such a devise has yet to be detonated anywhere, but the al-Qaida network is reported to have been interested in trying such a terror weapon. The worry is not of mass and immediate deaths, as in the 2001 attacks, but of the spread of radiation that might cause immediate panic because of long-term fear of illness. It might also make a section of a city uninhabitable for years.

"We know now there is no weapon they will not use, and no weapon they will not seek to acquire," Abraham said. "It is our responsibility to determine how to prevent such an attack in the first place, and how to respond if, despite all our best efforts, such an attack occurs anyway."

He noted that the U.S. government and IAEA last year announced a joint effort with Russia to secure radiation sources, particularly "orphaned" materials, in the former Soviet Union.

After that federation collapsed a decade ago, the ex-Soviet military and government abandoned unknown numbers of radiation sources in former Soviet states, including, for example, highly radioactive strontium-90 batteries used for remote aviation beacons.

Abraham announced that Washington was now ready to spend money and lend its expertise to expand the search for high-risk materials to other nations.

"It is my hope that this model, which is working so well in the former Soviet Union, will become global in scale," he told conference participants.

The more than 600 technical specialists, customs and other law enforcement officers, regulatory officials and others gathered here have an ambitions agenda, discussing ways to identify the most threatening radiation sources, how to find "orphaned" radioactive material, keeping track of sources in use, combatting smuggling, and emergency response to such an attack.

A recent U.S. experts' report concludes that worldwide "several tens of thousands" of the most dangerous radiation sources — used to treat cancer, find oil deposits, disinfect food — may be insufficiently protected.

In one of the first comprehensive studies of the threat, the researchers at California's Monterey Institute of International Studies noted problems in the United States, particularly lack of controls on U.S. exports of radioactive material.

Powerful radiation sources can be shipped from the United States to such terrorism-afflicted nations as Afghanistan and Colombia without any certification that the end user is legitimate and will protect the material from theft or misuse, they said in their January report. Stronger export controls in the United States and other countries could go far toward ensuring most high-risk radiation sources are safeguarded, they said.

Internal controls can also be strengthened. The U.S. government reported last year that 1,500 radiation sources were believed lost or stolen in the United States since 1996. ElBaradei says "cradle-to-grave" control of powerful sources is needed.

In Washington, Sen. Hillary Rodham Clinton, D-N.Y., and Rep. Edward Markey, D-Mass., have introduced legislation to establish a nationwide system of tracking radiation sources.

Abraham said he expects a report soon from a U.S. task force investigating such questions. "We felt a need to re-examine whether we're doing a proper job of tracking, accounting," the energy secretary said.