The director of One Direction's upcoming debut film has lavished praise on the band, calling them a "phenomenon".

The 'Kiss You' group are due to unleash the first ever documentary movie This Is Us in the UK from August 29 and it sees Super Size Me man Morgan Spurlock on directing duties.

Speaking to the BBC, the US documentarian said that One Direction's rise to fame over the past few years means that the film was an "incredible" opportunity for him. He said:

"They are a phenomenon. Right at this time now, their rocket is roaring through space. I'm just looking in through a window, and things have changed for them so quickly. In the last three years everything has transformed for them, and they're still just teenagers. So to have a snapshot of that moment is an unique opportunity for any director. And it's been incredible for me, never mind them."

Speaking meanwhile about the group on a personal level, Spurlock added:

"They're incredibly grateful and very thankful for what they have. And they really recognise that it's the fans who've made them who they are, after being the runners up on X-Factor. They're beautiful guys. I think I learned a lot just by being with them."