"There's obviously a high level of sophistication in the terrorist community," ElBaradei told reporters while flying back to Vienna, where the International Atomic Energy Agency (IAEA) is based, from a trip to Washington.
"That heightens the sense of concern that they (terrorists) might get their hands on any nuclear device or nuclear material," ElBaradei said in answer to a question about the implications of the train bombing in Spain on March 11 that killed 202 people.
The IAEA has repeatedly warned about the danger from terrorists possibly making so-called dirty bombs, conventional explosives laced with nuclear material that would not cause a chain reaction but would spread radiation and cause panic once they exploded.
ElBaradei had Wednesday urged in a meeting with US President George W. Bush for dangerous nuclear material such as highly enriched uranium used in civilian programs to be recycled or disposed of.
"I think any nuclear material that could be used in pure form or in crude form is dangerous enough," he said.
"One of the first priorities that I put to President Bush and he fully agreed is that we need to clean up all the nuclear materials that lie around, either in highly enriched uranium in research reactors or in fabrication facilities," ElBaradei said.
"I would like to see a civilian cycle completely free from weapons-useable material if possible," he said.
ElBaradei said he had found in his meetings in Washington this week, which also included national security advisor Condoleezza Rice and Energy Secretary Spencer Abraham, "a commitment in the United States at all levels to work in partnership with the agency (IAEA), meaning with the international community, to fight this new menace which we are facing, which is (an international nuclear materials) black market and the interests of terrorists to get their hands on nuclear technology."
ElBaradei had said in Washington after meeting Abraham that the US government was working on an "action plan . . . to clean up all the highly enriched uranium (HEU) and plutonium that is still in the civilian cycle."
This includes 100 facilities using highly enriched uranium -- over 21 of which are research reactors and the rest fuel production plants -- spread over 40 countries, ElBaradei said.
HEU can be used to make an atom bomb but also as fuel in reactors.
ElBaradei said Friday: "We have first to make an inventory (of the facilities and what is in them)."
He said the US Department of Energy was already working on this and that the IAEA may take part.
"Then we have to contact the countries (involved) . . . in order to take this material and neutralize or dilute it," ElBaradei said.
The IAEA is now overseeing a reactor in Libya from which highly enriched uranium was taken to Russia, which is to return it as low enriched uranium, which cannot be used in a bomb.
ElBaradei said he and Bush had also agreed the time had come to "change many of the rules" in order to strengthen the fight against nuclear proliferation that is the mission of the IAEA.
One measure would be to improve export controls "as a result of A.Q. Khan associates and the lesson we have learned from that," ElBaradei said.
Abdul Qadeer Khan, the father of Pakistan's nuclear bomb, confessed in January to running an international black market ring that shared sensitive nuclear technology with Iran, Libya and North Korea for more than a decade.