One Direction star Niall Horan has suggested he's battling pneumonia after falling ill just before Christmas.

The singer took to Twitter on Monday (19Dec16) to reveal he wasn't feeling his best, admitting the timing wasn't great.

"Get a chest infection Christmas week....," he wrote. "cool cool cool ........ not ideal".

His tweet prompted pop pal Ashton Irwin, from boyband 5 Seconds of Summer, to joke about how Niall contracted the illness when he spotted the post on Tuesday (20Dec16).

"Me too man, we shouldn't of (sic) kissed," he quipped. "I was highly contagious."

Niall responded, "You really shouldn't have. Ruined my Christmas."

Despite the light-hearted banter, the 23-year-old later went on to admit his medical diagnosis was not looking good: "I thought it was a chest infection and it's not, it's f**king worse," he added.

Niall has since hinted his illness is actually pneumonia, although he has yet to confirm the seriousness of his condition.

Returning to Twitter on Wednesday (21Dec16), he shared, "Should have listened to my mother years ago when she said ' don't go out with wet hair or you'll get pneumonia'".

The timing couldn't be worse for Niall as he is booked to perform on New Year's Eve. The Irish star, who is currently promoting his first solo single This Town, is set to hit the stage on 31 December (16) at a party in Los Angeles hosted by Fergie. Fifth Harmony, Ty Dolla $ign, John Legend, Lukas Graham, and Shawn Mendes are also on the bill for the big end of year bash, the California leg of the TV special Dick Clark's New Year's Rockin' Eve with Ryan Seacrest.

Seacrest will be presenting live from New York's Times Square, where Mariah Carey, DNCE, Thomas Rhett, and Gloria Estefan and the cast of her Broadway musical, On Your Feet!, will be performing.