Texas attack inspired but not directed by IS: Ashton Carter

An attempted attack on a controversial cartoon contest concerning Prophet Mohammed in the US state of Texas was inspired -- but not ordered by -- the Islamic State group, US Defence Secretary Ashton Carter said today.

IS jihadists had claimed credit for Sunday's failed attack in which two gunmen armed with assault rifles were shot dead by a police officer before they could storm the event in a Dallas suburb.

"Our understanding from the investigations that are going on was that these were inspired by ISIL, not directed by ISIL, which is an important distinction," Carter told reporters, using an alternative acronym for the extremist group.

"Still in all it's concerning that there are individuals like this who draw their inspiration from ISIL."

It was the first time the IS group, which has seized large swathes of territory in Iraq and Syria, had tried to take credit for an attack on US soil.

Carter said the effect of the group's online propaganda in the Texas attack provided "yet another reason why the defeat of ISIL important."

The White House had said earlier this week that intelligence and law enforcement agencies were still investigating whether IS was behind the attack in Garland, Texas.

One of the gunmen, Elton Simpson, was sentenced to three years' probation in 2011 after lying to the FBI about plans to travel to Somalia. Prosecutors alleged he was planning to travel there to wage holy war, but a judge dismissed the charge.

Simpson, 31, who had converted to Islam as a teenager, was in frequent contact with an American supporter of the IS group, according to terror experts.

Mohamed Abduallhi Hassan, an American who joined the IS group and who may be fighting in Somalia, had communicated with Simpson via Twitter.