Fritz, a polar bear cub at the Tierpark Berlin zoo, was struck down by a mysterious illness on Monday, with zookeepers now worrying for his life.

The four-month-old cub was found listless next to his mother Tonja, the zoo said on Monday.

"We very much hope that the little polar bear will get well soon and survive the night," Berlin Tierpark director Andreas Knieriem said. "We are all very worried."

The Berlin zoo shared photos of Fritz being examined by veterinarians at the nearby Leibniz Institute for Zoo and Wildlife Research on Twitter, writing that he is "very sick."

After hours of x-rays, ultrasound tests and CT scans, veterinarians still couldn't find any concrete evidence of what was ailing the young cub. The results from a fecal sample are still pending.

"Fritz has a massive liver inflammation that points to a serious illness," Knieriem explained. The polar bear cub has been given special antibiotics and painkillers.

Fritz was already behaving more lethargically on Sunday, but his condition had stabilized later on in the evening.

The cub, born on November 3 last year and only recently dubbed Fritz, is Tierpark Berlin's first baby polar bear in 22 years. He was supposed to be presented to the public for the first time in March.

Back in 2006, a polar bear cub named Knut born in another Berlin zoo stole German hearts and went on to become a world-renowned as he was hand-reared by a keeper after being rejected by his mother. Knut died in 2011 from an autoimmune disease at the age of four. In the wild, polar bears tend to live longer than 20 years.

rs/msh (AFP, dpa)