WASHINGTON – The terrorism threat to the United States over the next five years will be driven by instability in the Middle East and Africa, persistent challenges to border security and increasing Internet savvy, says a new intelligence assessment obtained by The Associated Press.

Chemical, biological, radiological and nuclear attacks are considered the most dangerous threats that could be carried out against the U.S. But those threats are also the most unlikely because it is so difficult for al-Qaida and similar groups to acquire the materials needed to carry out such plots, according to the internal Homeland Security Threat Assessment for the years 2008-2013.

The al-Qaida terrorist network continues to focus on U.S. attack targets vulnerable to massive economic losses, casualties and political "turmoil," the assessment said.

Earlier this month, Homeland Security Secretary Michael …