Niall Horan has spoken about undergoing "major" knee surgery in the US.

The One Direction star tweeted that his operation was a success and that he is now recovering.

Horan posted: "Hey guys I know most of you have seen those pics today! Yes I've had major knee surgery, a much larger surgery than first anticipated.

"But i would like to tell u, it was a complete success! Considering the scale of it."

He added: "All I ask of you guys is that you would respect my privacy as I try to recover... for the Where We Are tour."

One Direction are due to tour later this year, with their UK dates starting in May.