'Dirty bomb' within al-Qaeda's reach

In an annual report to Congress on proliferation threats, the US Central Intelligence Agency also repeated Tuesday its insistence that Iran was pursuing "a clandestine nuclear weapons program."

But it remained silent about charges made earlier this month by Secretary of State Colin Powell, who accused Iran of seeking to adapt its missiles to carry nuclear warheads.

Instead, the agency used its strongest terms to alert lawmakers to the threat of terrorist organisations using chemical, biological, radiological and nuclear materials to harm the United States and its allies, saying the danger of such an attack "remained high."

"One of our highest concerns is al-Qaeda's stated readiness to attempt unconventional attacks against us," the report pointed out.