New York police expand dirty bomb security

While security experts believed the threat of a global nuclear war had decreased, the risk of a one-off radiological or nuclear attack had increased, Riggio said.

Browne said small-scale nuclear devices were harder to make so a dirty bomb was a more likely pick for militant groups.

"We still don't have an adequate strategy of detecting nuclear material," New York Police commissioner Ray Kelly said at the conference. Browne said Kelly was referring to inadequate security at the country's ports, where radioactive materials could be smuggled in through large cargo containers.

(Editing by Doina Chiacu)