WASHINGTON - Still reeling from attacks in Brussels and Paris, world leaders are wrestling this week with the chilling prospect of the Islamic State group or other extremists unleashing a nuclear attack on a major Western city.

Preventing terrorists from obtaining nuclear materials is the central focus as President Barack Obama hosts leaders from roughly 50 countries for a nuclear security summit starting Thursday. Despite three previous summits and six years of Obama's prodding, security officials warn that the ingredients for a nuclear device or for a "dirty bomb" are still alarmingly insecure.

Leaders scheduled a special session during the summit on the threat to urban areas from extremist groups. U.S. officials say the leaders will discuss a hypothetical scenario about a chain of events that could lead to nuclear terrorism.