Policing the Internet, which jihadists are using with increasing sophistication to spread their gospel of terror across the world, is a challenge for the US, a top counter-terrorism official has said.

Acknowledging that monitoring the Internet was a challenge for counter-terrorism authorities to prevent another 9/11, Michael Leiter, Director of the National Counter-terrorism Centre (NCTC) says the US is doing all it can to disrupt jiadist websites.

While Leiter did not comment on specific operations, he said, "... all of what we do in the war on terror has to be all elements of national power."  "And part of that clearly can involve watching what jihadists are doing on the Internet and, when ... necessary, to disrupt the attacks, disrupting their ability to communicate, train and plot."

When it comes to disrupting terrorists' abilities to use terror for propaganda, Leiter, who will step down on Friday from NCTC, says it was a tricky area in the US because of the right to privacy and constitutional elements like the First Amendment and other legal aspects.

"So we are not there to stop people from communicating. We are there to disrupt plots," he was quoted as saying by CNN. "I think as the threat changes, as American expectations of privacy change, we have to constantly re-evaluate that.

"But again, as al-Qaeda evolves, as our expectation of privacy evolves, this has to be a constant review of what we are doing because we have to have the American people's trust to do it well," added Leiter.

Leiter said one of the the most striking trends during his 4’ years' tenure at the NCTC has been the proliferation of homegrown terrorist plots.

One of the best known cases of homegrown terrorism in the US is that of Major Nidal Hasan, the US Army psychiatrist charged with killing 13 people and wounding 32 others in a November 5, 2009 shooting rampage at the Fort Hood army base.

Last month, two US men were charged with plotting to attack a military centre in the northwestern US city of Seattle with machine guns and grenades, allegedly hoping to kill more people than Hasan did at Fort Hood.

Leiter said he believes rooting out the problem requires broader and deeper engagement with the Muslim community. "Like any social phenomenon there's not a single root cause. There's ideological pieces. There's psychological pieces; there's demographic pieces," he said.

"So the first is understanding it. And I think we've come a long way in understanding radicalisation here and abroad. Really most important, I think, (is) making sure that Americans understand that the American Muslim community is part of the solution to combating radicalisation, and not part of the problem," Leiter said.