Extremists used the controversial film "Innocence of Muslims" as a pretext to carry out worldwide attacks, according to US President Barack Obama.

"What we do know is that the natural protests that arose because of the outrage over the video were used as an excuse by extremists to see if they can also directly harm US interests," Obama said.

Speaking at a candidate forum on Spanish-language network Univision, Obama said the attack that killed Ambassador Chris Stevens and three other Americans in Benghazi is under investigation, adding that it was still unclear if al-Qaeda was responsible.

The amateur film, which portrays Prophet Muhammad as a religious cheat and a womanizer,has sparked widespread protests across the Middle East and North Africa, some of which have turned violent.

Last week, an attack on the US consulate in Benghazi, said to be triggered by the film, killed Ambassador Christopher Stevens, 3 other officers, and several Libyans.

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