President Obama has called nuclear security a top national security priority, and he and other world leaders pledged cooperation against the risk of proliferation from outlaw nations such as North Korea during a nuclear security summit in March.

Radioactive material is used in diagnosing and treating cancer and other diseases. In hospital settings, it is usually encased in metal. There has been no known terrorist theft of nuclear materials from medical facilities.

Still, the terrorism risk has focused on the use of radioactive material to build rough bombs that could cause widespread economic damage and panic if detonated in a subway or high-rise building as well as more sophisticated “suitcase” bombs that could be more powerful.

Al-Qaeda has long sought weapons of mass destruction, though its actions have focused on more conventional bombs.

More than a decade after it launched the attacks on the World Trade Center and the Pentagon, al-Qaeda is described by counterterrorism experts as a significantly weakened organization. The disruption of an underwear bomb plot developed in Yemen this year underscored the organization’s vulnerability as much as its dedication to continuing to develop plots against U.S. targets.

But officials and experts have voiced growing concern about al-Qaeda’s ability to exploit instability in the Middle East to forestall its demise at the same time that the turmoil has complicated counterterrorism relationships for the United States.

Insurgencies in Syria and elsewhere have made al-Qaeda “operationally active in an environment in which their growth and activity is not being checked,” said Juan Zarate, who was a counterterrorism adviser in the George W. Bush administration.

Al-Qaeda’s affiliate in Yemen is the most potent threat to the United States, officials said, because it remains committed to trying to hit U.S. targets in creative ways.

“The threat is still high out of Yemen, and it’s a really dynamic time in places like Egypt, Syria and across North Africa,” a U.S. counterterrorism official said.

Greg Miller contributed to this report.