United States will face a persistent and evolving terrorist threat over the next three years, including attacks by terror groups on behalf of Iran, according the the National Intelligence Estimate released today.

The report by Director of National Intelligence identified Islamic terrorist groups and cells, especially al-Qaeda, as the main threat but warned that Lebanese Hizballah, which has conducted anti-US attacks outside the United States in the past, may be more likely to carry out terror strikes on American soil over the next three years on Iran is threatened.

The NIE blamed the Internet for helping small number of radicalised Muslims to find and connect with one another to carry out terror strikes on inside the US without requiring a centralized terrorist organization, training camp, or leader.

The Intelligence Officials noted that the radical and violent segment of the West’s Muslim population is expanding, including in the US, as evidenced by the spread of radical Salafi Internet sites, increasingly aggressive anti-US rhetoric and actions, and the growing number of radical, self-generating cells in Western countries.

The report warned against complacency even though tough new counterterrorism measures have constrained the ability of al-Qaeda to attack the again.

"We are concerned, however, that this level of international cooperation may wane as 9/11 becomes a more distant memory and perceptions of the threat diverge," the authors stated.

According to the NIE, al-Qaeda "will continue to try to acquire and employ chemical, biological, radiological, or nuclear material in attacks and would not hesitate to use them if it develops what it deems is sufficient capability."

"The ability to detect broader and more diverse terrorist plotting in this environment will challenge current US defensive efforts and the tools we use to detect and disrupt plots," the estimate concluded. "It will also require greater understanding of how suspect activities at the local level relate to strategic threat information and how best to identify indicators of terrorist activity in the midst of legitimate interactions."