'Like a sound from hell': Serbia and sonic weapons

Pressure is building on the Serbian government after reports that it used an illegal sonic weapon on protesters in Belgrade.

Tens of thousands of people were holding a peaceful anti-corruption rally when "we heard something we could not see", said protester Ivana Ilic Sunderic, "ike a sound rolling toward us". It was "very unusual and very frightening, like a sound from hell", she told Sky News.

Serbian authorities denied the use of an acoustic weapon. But as video footage emerges of the incident...