An Australian Twitter user and Islamic State supporter who appeared to encourage the terrorist attack on an anti-Islamic cartoon event in the US says his movement is "winning the minds of the young generation".

The man, who goes by the Twitter name Australi Witness, told Fairfax Media he supported what "our mujahideen [holy warriors] in Texas did" but denied that he specifically told them to launch the attacks, in which both assailants died and one security guard was wounded.

The Perth-based man's activities underscore how social media has created a global reach for jihad sympathisers who can inspire violence across great distances without ever personally knowing the people they are inciting.

"This is a war that will be won through the power of social media," the man said. "We are winning the minds of the young generation."