Niall Horan, Harry Styles, Zayn Malik, Louis Tomlinson and Liam Payne of One Direction. (WENN.COM)

One Direction star Liam Payne is concerned his bandmate Zayn Malik is struggling to come to terms with fame, insisting the pop star has had a "terrible time" facing criticism online.

The boyband shot to fame in the U.K. on Simon Cowell's The X Factor in 2010, and have gone on to become a global sensation, catapulting all five members into the limelight.

Payne insists most of the group love the attention they receive, but he worries about Malik, who recently temporarily deleted his Twitter.com account after reading a slew of nasty messages.

Payne tells Britain's The Sun, "We've (the band) had such a good laugh together and achieved so much together. We're definitely like brothers.

"The only person I really worry about is Zayn, because I don't think he's quite accustomed to all of this. He had a terrible time on Twitter... I don't think he's quite adjusted to it yet. I'm sure he'll get it. He's my best mate. I've got to look out for him."