Almost nine years after the Sep 11 2001 terror attacks, a worrying number of American citizens and longtime US residents are becoming radicalised and training to be terrorists in Pakistan, US counter-terrorism officials say.

Brainwashed by Al Qaeda's extremist ideology, many are also learning the bomb-making skills necessary to become potentially dangerous terrorists, CNN said in a special report citing officials. They are training in the mountains of Waziristan in northwestern Pakistan, where Al Qaeda still enjoys significant safety, it said.