Air Force Gen Gene Renuart, who is chief of the US Northern Command, told reporters yesterday he has not seen any direct threats tied to the US presidential elections. But he said it would be imprudent to think that such threats are not there.

"We need only to look at Spain and see that they're certainly willing to try to do something that is significant that could effect an election process," Renuart said. "I think it would be imprudent of us to let down our guard believing that if there's no credible threat that you know of today, there won't be something tomorrow."

While he said that US authorities have thwarted attacks on a number of occasions, he said terrorist cells may be working harder than ever to plot high-impact events. He did not point to any specific intelligence that authorities have received, but said the "chatter" they are hearing "gives me no reason to believe they're going to slow down" in their efforts to target the United States.

"If an organisation like that is to maintain credibility and continue to grow more of its extremists, it has to show tangible results," Renuart said. "So I think there may be a certain sense of urgency among that organisation to have an effect. So it would tell me that they're trying harder."