Pop star Taylor Swift struggled to sing her hits as she battled laryngitis at two festive concerts.

The Shake It Off hitmaker performed at the annual KIIS FM Jingle Ball at the Staples Center in Los Angeles on Friday (05Dec14), but warned fans ahead of the show that her voice was not at its best.

She wrote on Twitter.com, "I'm about to go onstage at... KIISJingleBall in La. Fun fact: I have a wicked case of laryngitis. This should be interesting!"

Despite her illness, Swift flew to London to headline the Capital FM Jingle Bell Ball on Sunday (07Dec14), and issued a second warning to her fans, writing, "The voice is feeling hella (really) bad but I'm here and ready to Party/sound like a frog."

She also thanked fans for their support while onstage, telling them, "I do have a severe case of laryngitis which is making it hard to sing, but you're helping me... I want to thank you for staying and hanging out with me."