The United States is deliberately sparring with Al-Qaeda supporters and militants online aiming to shoot down extremist messages and win over hearts and minds, a US official said Wednesday.

Seeking out the virtual spaces where "Al-Qaeda and its supporters lurk" is part of America's strategy to combat violent extremism, Under Secretary for Public Diplomacy and Public Affairs Tara Sonenshine told students at the University of Maryland.

"We robustly engage with them in chat forums in Arabic, Somali, Punjabi and Urdu," she explained.

"By targeting the hardliners, we are really trying to reach the middle grounders, the fence sitters, the sympathizers and passive supporters."

Last year, staff at the new Center for Strategic Counterterrorism Communications, which reports directly to Sonenshine, wrote some 7,000 posts on different online forums.

"When we counter unfounded rumors, propaganda and conspiracy theories with facts, truth and reasonable argument, we can better define what we stand for. And we have a greater chance of changing more minds," Sonenshine said.

"We cannot afford to stand on the sidelines and let cynical voices distort and misreport our policies, values, and interests," she added.

"We have to enter that communications space -- where so many young people are getting their information -- and contest extremist and other negative rhetoric with facts and counter-assertions."

She admitted it was hard to assess whether such virtual online battles were helping to win the war against extremists, quipping that "terrorists are not exactly willing to participate in surveys."

But she said it was considered "a gain" when Al-Qaeda complained about the center's activities in their online forums or when a Saudi citizen called on his government to set up such a center.

Pressing the case for such continued public diplomacy, Sonenshine argued: "What is the cost of not confronting negative messaging?"

"Are we really going to leave it to our adversaries to define America, and recruit the disenfranchised and the impressionable to their cause?" she asked.