The head of the U.N. nuclear watchdog agency said today that stricter security measures were urgently needed to keep radioactive material out of the hands of terrorists, who could use it to spread havoc with "dirty bombs."

Mohamed ElBaradei, the International Atomic Energy Agency chief, made the call on the opening day of an international conference on dirty bombs, which are made by attaching radioactive material to a conventional bomb to spread it over a wide area.

ElBaradei said the Sept. 11, 2001, attacks in the United States showed that terrorists would not be afraid to handle deadly radioactive material to construct such a bomb.

"Given the apparent readiness of terrorists to disregard their own safety, the personal danger from handling powerful radioactive sources can no longer be seen as an effective deterrent," he said.

While there has never been a dirty bomb attack, ElBaradei said recent reports about terrorist plans to use dirty bombs were worth taking seriously and that countries should spend time and money to beef up nuclear security. "Our database of cases of smuggling . . . gives an indication that there is a market and there is an effort to obtain radioactive sources, and the obvious question is why," he said.

Britain said in January it had evidence that Osama bin Laden's al Qaeda network, blamed by the United States for the Sept. 11 attacks, tried to develop a dirty bomb in the late 1990s.

But some counterterrorism experts and even officials within the IAEA argue that such bombs are generally of little interest to such groups as al Qaeda because they are less effective than more easily accessible weapons.

Russia's atomic energy minister, Alexander Rumyantsev, told reporters that Russian security services had "no indication that terrorists have come to possess dirty bombs."

ElBaradei said that though a dirty bomb might not necessarily kill its victims, the most severe impact would be "panic and social disruption associated with exposure to radiation, the very purpose of an act of terror."

There have been more than 280 confirmed cases of criminal trafficking of radioactive material, though the IAEA chief said, "The actual number of cases may be significantly larger than the number reported to the agency."