More action is needed to stop militants acquiring plutonium or highly-enriched uranium that could be used for atomic bombs, nuclear experts and government officials said.
Speaking in Vienna on Monday, Yukiya Amano, head of the International Atomic Energy Agency (IAEA), warned against a "false sense of security" over the danger of nuclear terrorism.
Amano, holding up a small lead container that was used to traffic highly enriched uranium in the former Soviet republic of Moldova two years ago, said it showed a "worrying level of knowledge on the part of the smugglers".
"This case ended well," he said. "Unfortunately, we cannot be sure if such cases are just the tip of the iceberg."
Analysts said radical groups could theoretically build a crude, but deadly nuclear bomb if they had the money, technical knowledge and the amount of fissile material needed.
Many states have taken steps to prevent malicious acts such as nuclear theft and sabotage, Amano told the delegates.
"Partly as a result of these efforts, there has not been a terrorist attack involving nuclear or other radioactive material," he said. "But this must not lull us into a false sense of security."
Obtaining weapons-grade fissile material - highly enriched uranium or plutonium - poses the biggest challenge for militant groups, so keeping it secure is vital, experts say.
Because radioactive material is seen as less hard to find and the device easier to manufacture, experts say a so-called "dirty bomb" is a more likely threat than a nuclear bomb.
In a dirty bomb, conventional explosives are used to disperse radiation from a radioactive source, which can be found in hospitals or other places not very well protected.
US Energy Secretary Ernest Moniz repeated Washington's assertion that al-Qaeda had been trying for years to obtain nuclear material for a weapon. "We should expect its adherents … to continue trying to achieve their nuclear ambitions," he said.
More than 100 incidents of theft and other unauthorised activities involving radioactive material were reported to the IAEA every year, Amano said.
"Some material goes missing and is never found," he said.