BERLIN - Germany's celebrity polar bear cub Knut has received an anonymous death threat, causing alarm at Berlin Zoo on Thursday and prompting heightened security.

©REUTERS/Arnd Wiegmann
Polar bear cub Knut shakes off water at an enclosure in Berlin zoo.

Top-selling Bild newspaper said the zoo had received a hand-written fax from a suspected animal hater with the words: "Knut is dead! Thursday midday."

But that deadline came and went safely for media star Knut, who has been on newspaper front pages around Germany and the world for weeks. "He is safe and in good spirits," said zoo official Ragnar Kuehne after the time had passed.

Berlin police said they had investigated a letter containing a threat but did not believe it was serious.

Berlin Zoo's business manager Gerald Uhlich said: "They told us prominent figures often have things like this happen but in this instance we need not be too worried."

Nonetheless, Bild said the zoo had trebled the number of minders responsible for Knut's safety to 15.

"Death threat, police protection for Knut!" Bild wrote in bold letters on page one above a picture of the sad-looking polar bear cub peering out from behind a tree branch.

As Knut appeared for one of his public appearance on Thursday, about twelve minders in orange jackets and carrying walkie-talkies patrolled the area around his enclosure to keep a close eye on the cub and the crowd of fans.

Just an hour before the death threat expired, an unperturbed Knut rolled around on the ground with his bearded keeper Thomas Doerflein and, as usual, chewed his fingers.

Knut, who now has his own brand, won global attention after questions were raised about hand-rearing polar bears. Some animal rights campaigners were interpreted as calling for him to be put down rather than raised by humans.

Doerflein has slept in Knut's cage, played with him and fed him porridge since the cub's mother rejected him at birth.

The zoo has already received about six serious offers to give Knut a home when he is older, the latest from a Norwegian zoo north of the Arctic Circle.

"We are the northernmost zoo in the world so we thought, why not take the initiative to get him up here," said Sigbjoern Soerensen, managing director of the Polar Zoo in Bardu, Norway.

Soerensen, who sent his proposal to Berlin in a letter, said the Polar Zoo had ideal conditions for Knut, with deep snow in winter and temperatures sometimes diving to -30 degrees Celsius and summer temperatures typically a cool 10-15 degrees.

Kuehne said Berlin Zoo would only consider whether to send Knut abroad in a year or so when he is old enough to breed.

(Additional reporting by John Acher in Oslo)