Police shot dead two gunmen Sunday outside a Prophet Mohammed cartoon contest in Texas attended by controversial Dutch far-right politician Geert Wilders, authorities said.

While no immediate claim of responsibility was made, similar depictions of the Prophet Mohammed prompted a shooting at French satirical weekly Charlie Hebdo in January that killed 12.

US authorities are investigating the shooting and police said it was still unclear if the attack was related to the event.

The right-wing American Freedom Defense Initiative (AFDI) organized the event in a suburb of Dallas, featuring Wilders, who has been outspoken against Muslims and is targeted by radical groups.

Police said two men drove up to the conference center in Garland, Texas, and began shooting at a security guard.

"Garland Police officers engaged the gunmen, who were both shot and killed," the city of Garland said in a statement.

The security guard was shot in the ankle and was treated at a hospital and released, the city said.

Local police said the shootout lasted "seconds," and organizers said they had prepared extra security for the event due to the general risk of an attack.

Wilders has also long been targeted by Islamists because of his extreme views on Islam.

"I am shocked. I just spoke for half an hour about the cartoons, Islam and freedom of speech and I had just left the premises," Wilder told AFP in an email.

"This is an attack on the liberties of all of us!" Wilders wrote, adding: "I hope it is not connected to death list (of) Al-Qaeda." He added that he was safe with police.

The Dutch politician said he is returning to the Netherlands Monday but would come back to the United States next week for another speaking engagement.

Prophet Mohammed cartoons

Many Muslims find depictions of the Prophet Mohammed offensive and such cartoons have triggered violent protests. The Danish daily Jyllands-Posten published 12 satirical cartoons in 2005, triggering deadly protests in some Muslim countries.

Cartoon images of the Prophet Mohammed were also published in French satirical weekly Charlie Hebdo, where Islamist gunmen killed 12 people in January.

The identities of Sunday's shooters has yet to be confirmed, but the SITE Intelligence Group reported that an Islamic State (IS) fighter claimed on Twitter that the shooting was carried out by two pro-IS individuals.

In a series of tweets and links, a jihadist named as Abu Hussain AlBritani, which SITE said was British IS fighter Junaid Hussain, claimed that "2 of our brothers just opened fire" at the Prophet Muhammad exhibition in Texas.

"They Thought They Was Safe In Texas From The Soldiers of The Islamic State," added the tweet.

About 200 people were inside the event, said local police spokesman Police spokesman Joe Harn.

He said the first suspect was shot dead immediately, while the second was shot after reaching for his backpack.

Police said they suspect the gunmen's vehicle may contain an "incendiary device" and a bomb squad was on the scene.

"The bodies are still outside near the car. Once that car is cleared then we'll take care of the bodies," he told reporters.

AFDI, criticized for promoting anti-Islamic views, offered a $10,000 prize for the winner of the cartoon contest that was billed as a "free speech" event.

AFDI co-founder and political activist Pamela Geller called the shootings a "war on free speech."

"What are we going to do? Are we going to surrender to these monsters?" she wrote on her website.

"The war is here."

Speaking to Fox News, she added: "The idea we are going to bridge our freedom, our most basic inalienable right in order to not offend savages is egregious, it is outrageous.