The number of worldwide terror attacks fell to 10,283 last year, down from 11,641 in 2010 and the lowest since 2005, the State Department reported today.

What's made the difference? The State Department cites the May 2011 killing of Osama bin Laden and other top al Qaeda members killed last year including Atiyah Abd al-Rahman and Anwar al-Awlaki, who was the head of Yemen's Al Qaeda affiliate and had ties to the underwear bomber plot in 2010.

"The loss of bin Laden and these other key operatives puts the network on a path of decline that will be difficult to reverse," the report stated.

But Ambassador Dan Benjamin, the State Department's coordinator for counter-terrorism, warned that for all the good news about the core of al Qaeda being weakened, affiliates of the group, particularly in Yemen and in Africa, continue to pose a real risk.

Nigeria was one of the few countries which actually saw an increase in terror attacks last year because of Boko Haram, and Kenya and Somalia continue to experience attacks by a weakened Al Shabab. Benjamin also noted that the Arab spring and other countries in transition could leave important allies like Egypt and Iraq vulnerable to terror groups.

"Inspiring as the moment may be, we are not blind to the attendant perils. Terrorists could still cause significant disruptions for states undergoing very challenging democratic transitions. Affiliates of the group, and violent extremist ideology and rhetoric continue to spread in some parts of the world," said Benjamin.

Reports of al Qaeda operatives taking advantage of the instability in Syria is also a potential worrying situation, says Benjamin. The U.S. has warned Syria's opposition groups against allowing foreign fighters to join the resistance, and Benjamin says opposition groups have assured U.S. officials that they are being vigilant in keeping extremists out. But he placed the blame for the conflict squarely on Syria's President Bashar al Assad.

"So long as Assad refuses to go and Syria's transition is blocked, the danger grows of more foreign fighters, including extremists of the al Qaeda type, infiltrating Syria, " he said.

Though the report focuses primarily on the threat al Qaeda and its affiliates pose to the United States, the activities of Iran over the last year are also increasingly of concern, specifically Iran's support for Hezbollah and the rogue nation's involvement in the 2011 plot to assassinate Saudi Arabia's ambassador the United States in Washington, D.C.

Get More News at ABC Politics and a Different Spin at OTUS News