Obama frets over 'lone wolf' attack as Sept 11 looms

"The most likely scenario that we have to guard against right now ends up being more of a lone wolf operation than a large, well coordinated terrorist attack," he told CNN television during a campaign-style swing through Iowa.

"We still have to stay on top of it, though. We're never letting our guard now, that's part of our job," said Obama, who vowed "heightened" security measures and "extra vigilance" ahead of the grim anniversary.

He had been asked about the prospects of a terrorist attack either to mark 10 years since the strikes on the Pentagon and the World Trade Center or in retaliation for the May raid in which US commandos killed Osama bin Laden.

Obama said US officials were "constantly monitoring potential risks" but said a punishing US campaign had left Al-Qaeda "a much weaker organization with much less capability than they had just two or three years ago."

"The risk is always there, and obviously on a seminal event like the tenth anniversary of 9/11, that makes us more concerned -- it means we've got heightened awareness," he said.

But "the biggest concern we have right now is not the launching of a major terrorist operation -- although that risk is always there."

"The risk that we're especially concerned over right now is the lone wolf terrorist, somebody with a single weapon being able to carry out wide-scale massacres of the sort we saw in Norway recently," said Obama.