A nuclear attack by terrorists against the United States is inevitable, said an American international relations expert visiting the New Zealand University of Canterbury.

Last week US Attorney General John Ashcroft said credible intelligence from multiple sources indicated that the al Qaeda terror network was planning an attack on the US in the next few months.

Professor Terry Nardin, a political scientist from the University of Wisconsin in Milwaukee, said in statement Tuesday that despite the intelligence sources of the US authorities, terrorist organisations such as al Qaeda are still capable of planning and carrying out attacks.

"I see a kind of offence-defence race. We're busy defending ourselves in ways that we've thought of and al Qaeda people, who are smart, are busy thinking of ways to get around it," he said.

Nardin said there are so many weapons floating around, there isso little control over nuclear material, biological weapons and soforth, "I think it's only a matter of time before there is some catastrophic event."

Nardin believed it is not difficult to transport nuclear materials or biological materials around.

"There may be a gesture of inspecting aircraft luggage and so on but the whole world container trade goes on so people move around and things move around," he said.