Washington, Aug 18 (IANS): US Secretary of Homeland Security Janet Napolitano has announced new steps to guard against terror attacks ahead of the 10th anniversary of the 9/11 attacks, calling on citizens to be more alert to possible terrorist threats.

Napolitano Wednesday announced an expansion of the department's "If you see something, say something" advertising campaign, and encouraged the reporting of suspicious behaviour.

The campaign would have many new television and radio ads, along with announcements in public buildings and events, Xinhua reported.

The secretary said the campaign was aimed at increasing public participation in preventing terror attacks.

The country faces less risk from massive terror strikes than from "smaller but harder-to-detect" attacks, she said.

"What we see instead are smaller plots involving fewer people so they are much more difficult to intercept," Napolitano said.

"We are also seeing the rise of the activity by individuals who are actually in the country and they are acting by themselves... That kind of attack is the most difficult to prevent because there is nothing to intercept."

President Barack Obama has also warned against the threat of a "lone wolf" terror attack.

"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 said.

Obama said such an attack would look like what took place in Norway, when one person killed dozens of people without elaborate organisational support.