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, a new intelligence assessment says.

Chemical, biological, radiological and nuclear attacks are considered the most dangerous threats that could be carried out against the United States. But those are also the most unlikely, because it is so difficult for al-Qaeda 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 Associated Press obtained a copy of the report.

Al-Qaeda continues to focus on U.S. attack targets that are vulnerable to huge economic losses, casualties and political "turmoil," the assessment says.

Earlier this month, Homeland Security Secretary Michael Chertoff said the threat posed by weapons of mass destruction remained "the highest priority at the federal level."

Speaking to reporters Dec. 3, Chertoff said more people would learn how to make dirty bombs and biological and chemical weapons.

"The other side is going to continue to learn more about doing things," he said.

The threat report, marked "for official use only," does not specify its audience, but the assessments typically go to law enforcement, intelligence officials and the private sector.

When determining threats, intelligence officials consider loss of life and economic and psychological consequences.

Intelligence officials also predict that in the next five years, extremists will try to conduct a destructive biological attack. Officials are concerned about the possibility of infections to thousands of U.S. citizens, overwhelming regional health-care systems.

Workers' illnesses and deaths could also have dire economic implications. Officials are most concerned about biological agents, such as anthrax, being stolen from labs or other storage facilities.

"The threat of terrorism and the threat of extremist ideologies has not abated," Chertoff said in his year-end address Dec. 18. "This threat has not evaporated, and we can't turn the page on it."

These high-consequence threats are not the only kind of challenges that will confront the United States over the next five years.

Extremists will continue to try to evade U.S. border-security measures and place operatives inside the mainland to carry out attacks, the 38-page assessment said. It also said they may pose as refugees or asylum-seekers or try to exploit foreign travel channels such as the Visa Waiver Program, which allows citizens of 34 countries to enter the United States without visas.

Long waits for immigration and more restrictive European refugee and asylum programs will lead more foreigners to try to enter this country illegally. Increasing numbers of Iraqis are expected to migrate to the United States in the next five years, and the number of refugees from Somalia and Sudan could increase because of conflicts in those countries, the assessment said.

Because of a proposed cap of 12,000 refugees from Africa, officials expect more will try to enter the United States illegally as well. Officials predict the same scenario for refugees from Afghanistan, Bangladesh and Pakistan.

Intelligence officials predict that the pool of radical Islamists within the United States will increase over the next five years in part because of the ease of online recruiting methods. Officials foresee "a wave of young, self-identified Muslim 'terrorist wannabes' who aspire to carry out violent acts."

The Lebanese Shiite group Hezbollah has no known history of fomenting attacks inside the United States, but that could change if there is some kind of "triggering" event, the Homeland Security assessment cautions.

In addition, the cyber-terror threat is expected to increase, as hacking tools become more sophisticated and available.

"Youthful, Internet-savvy extremists might apply their online acumen to conduct cyber attacks rather than offer themselves up as operatives to conduct physical attacks," according to the assessment.

It said Al-Qaeda had the capability to hire sophisticated hackers to carry out these kinds of attacks.