GARLAND, TEXAS (AFP) - The Islamic State in Iraq and Syria (ISIS) claimed responsibility Tuesday via its official radio station for a gun attack on an anti-Muslim event in Texas over the weekend showcasing cartoons mocking the Prophet Muhammad.

"Two of the soldiers of the caliphate executed an attack on an art exhibit in Garland, Texas, and this exhibit was portraying negative pictures of the Prophet Muhammad," the radical group said.

"We tell America that what is coming will be even bigger and more bitter, and that you will see the soldiers of the Islamic State do terrible things," the group announced. It was the first time ISIS claimed to have carried out an attack in the US.

Police said two men drove up to the conference centre Sunday in Garland, Texas, where the right-wing American Freedom Defense Initiative was organising the controversial cartoon contest, and began shooting at a security guard.

Garland police officers then shot and killed both men. According to US media reports, the two suspected Islamists were Elton Simpson, 31, and Nadir Soofi, 34, who shared an apartment in Phoenix, Arizona.

Simpson was being investigated by the FBI over alleged plans to travel to Somalia to wage holy war, court records show.

Many Muslims find drawings of the Prophet to be disrespectful or outright blasphemous, and such cartoons have been cited by Islamists as motivation in several previous attacks.