A US filmmaker has been arrested for sending his 16-year-old nephew onto the street with a fake grenade launcher to test police response times.

Michael Turley, 39, allegedly filmed the teenager dressed in a sheet while pointing the realistic looking weapon at passing cars.

The video, which was posted on YouTube, appears to show the fake terrorist on a busy road in Phoeniz, Arizona, for 15 minutes before police arrive.

But Phoenix Police Department spokesman James Holmes said officers were on the scene within a minute of being alerted.

“They told us they were just making a movie,” Mr Holmes told the Arizona Republic, adding that there was no arrest that day.

“We deemed it a pretty dumb action but we didn’t know what their real intent was, so we initiated an investigation.”

Turley faces charges of giving a false impression of a terrorist act, endangerment and contributing to the delinquency of his minor nephew, 16, who also faces charges.

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