By Fredrik Dahl
VIENNA (Reuters) - More action is needed to prevent militants acquiring plutonium or highly-enriched uranium that could be used in bombs, governments agreed at a meeting on nuclear security in Vienna on Monday, without deciding on any concrete steps.
A declaration adopted by more than 120 states at the meeting said "substantial progress" had been made in recent years to improve nuclear security globally, but it was not enough.
Analysts say radical groups could theoretically build a crude but deadly nuclear bomb if they had the money, technical knowledge and materials needed.
Ministers remained "concerned about the threat of nuclear and radiological terrorism ... More needs to be done to further strengthen nuclear security worldwide", the statement said.
The document "encouraged" states to take various measures such as minimizing the use of highly-enriched uranium, but some diplomats said they would have preferred firmer commitments.
Many countries regard nuclear security as a sensitive political issue that should be handled primarily by national authorities. This was reflected in the statement's language.
Still, Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), which hosted the conference, said the agreement was "very robust" and represented a major step forward.
RADICAL GROUPS' "NUCLEAR AMBITIONS"
Amano earlier warned the IAEA-hosted conference against a "false sense of security" over the danger of nuclear terrorism.
Holding up a small lead container that was used to try to traffic highly enriched uranium in Moldova two years ago, the U.N. nuclear chief said it showed a "worrying level of knowledge on the part of the smugglers".
"This case ended well," he said, referring to the fact that the material was seized and arrests were made. But he added: "We cannot be sure if such cases are just the tip of the iceberg."
Obtaining weapons-grade fissile material - highly enriched uranium or plutonium - poses the biggest challenge for militant groups, so it must be kept secure both at civilian and military facilities, experts say.
An apple-sized amount of plutonium in a nuclear device and detonated in a highly populated area could instantly kill or wound hundreds of thousands of people, according to the Nuclear Security Governance Experts Group (NSGEG) lobby group.
But 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 that are generally not very well protected.
More than a hundred incidents of thefts and other unauthorized activities involving nuclear and radioactive material are reported to the IAEA every year, Amano said.
"Some material goes missing and is never found," he said.
U.S. Energy Secretary Ernest Moniz said al Qaeda was still likely to be trying to obtain nuclear material for a weapon.
"Despite the strides we have made in dismantling core al Qaeda we should expect its adherents ... to continue trying to achieve their nuclear ambitions," he said.
(Editing by Raissa Kasolowsky)