Terrorists flying drones to spread highly radioactive material over a civilian area: That's part of the nightmare scenario President Barack Obama urged world leaders to consider as they debated better ways of controlling nuclear material.

With the aid of apocalyptic fake newscasts, Obama told the group of 50 heads of state and foreign ministers in Washington Friday to imagine that a terrorist group had bought isotopes through brokers on the so-called dark Web. One shipment was picked up in transit by radiation monitors, but others were thought to be still on the move. The terrorists were believed to be planning to use a drone to distribute the material. Would authorities react in time?

This hypothetical situation was described by a U.K. official speaking on condition of anonymity because the meeting was closed. Obama's aim was to push the men and women around the table to think about how their governments would respond. British Prime Minister David Cameron told reporters the threat is serious.