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 two-day nuclear security summit starting today. Despite three previous summits and six years of Obama’s prodding, security officials warn that the ingredients for a nuclear device or a “dirty bomb” are alarmingly insecure.

“We know that terrorist organizations have the desire to get access to these raw materials and to have a nuclear device,” said Ben Rhodes, Obama’s deputy national security adviser.

Decades after the Cold War, the threat of a nuclear war between superpowers has given way to growing concerns about non-state actors, including Islamic State and al-Qaida offshoots operating in North Africa and in Yemen and Saudi Arabia. Although the U.S. and its allies still worry about North Korea, Obama believes the threat posed by Iran has subsided due to the nuclear deal, leaving extremist groups among the likeliest perpetrators.

The havoc such an attack could wreak in an urban area like New York or London is concerning enough that leaders scheduled a special session on the threat during the summit.