US Secretary of the Department of Homeland Security (DHS) Janet Napolitano on Wednesday announced new steps to guard against terror attacks ahead of the 9/11 anniversary next month, calling on its citizens to be more alert to possible terrorist threats.

Speaking at an event at the US Chamber of Commerce, Napolitano announced an expansion of the department's "If you see something, say something" advertising campaign, encouraging the reporting of suspicious behavior with a dozen new television and radio ads, along with announcements in public buildings and events.

The secretary said that the campaign is aimed at increasing public participation in preventing terror attacks as the country faces less risk from massive terror strikes than from smaller but harder-to-detect attacks.

"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."

The remarks echoed US President Barack Obama, who on Tuesday warned against the threat of "lone wolf" terror attack by saying " 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."

In an interview with CNN, Obama said that such attack would look like what took place in Norway last month, when one person killed dozens of people without elaborate organizational support.

The Chamber of Commerce is encouraging all its members to utilize the new public service announcements of the "If you see something, say something" campaign.

"A safe and secure environment is critical to every single business operating in the United States," said Tom Ridge, the former secretary of the DHS who was also present at the Chamber of Commerce event. "Businesses have a stake in this debate. When it comes to securing the homeland, federal, state, and local governments have no stronger partner than the private sector."