Two polar bears, Vera and Vilma, gave birth a few weeks ago. The zookeepers had decided not to disturb the cubs, even if the mothers rejected them.

Vilma scratched her feed box on Monday, "appearing nervous, well-fed, obviously not hungry," a zookeeper was quoted as saying. No cubs could be seen or heard.

So as not to disturb the cubs, the zoo did not check how many were born.

Zoo director Dag Encke said that Vilma's cubs - believed to be a pair - may have been ill and that in such cases polar bears often eat their young.

Vera was spotted briefly with one of her cubs

Vera, meanwhile, has appeared briefly with one of her cubs. Mr Encke's deputy Helmut Maegdefrau said the cub appeared "in perfect shape".

The zoo has said it wants to avoid any repetition of "Knut mania", a popular craze triggered by a hand-reared bear cub at Berlin Zoo last year.

Knut was rejected by its mother, but Berlin Zoo decided to hand-rear it - a move which proved enormously popular.

But Mr Maegdefrau, quoted by the German news website Focus, said "we should have faith in nature and accept that sometimes with new mothers things just don't work out.

"If we were to go in, we would disturb them and make it more likely that something goes wrong," he added.

Berlin Zoo said it disagreed with the Nuremberg zoo staff's decision.

Since they started hand-rearing Knut, he has become a celebrity - both in Germany and worldwide.