A mainland zoo has been forced to deny that its sun bear is actually a human in a costume, after footage of one standing on its hind legs raised online accusations of a furry imposter.

A video clip of a bear rearing up and interacting with a group of people at a zoo in eastern Hangzhou city went viral on social media.

Many users posted comments doubting that the bear was real, with some alleging that its bipedal posture and wrinkled skin suggested that it was actually a human in costume.

But the zoo dismissed the rumours, saying in a statement written from the bear's perspective on Sunday that the animal was real and its detractors "really don't understand me".

"The zoo director called me after work yesterday and asked if I'd been slacking off by finding a two-legged beast to replace me," the bear, named Angela, said in the statement.

"Some people thought the way I stand up looks too human... so I will stress again: I'm a sun bear!"

Native to Southeast Asia, sun bears are named for a patch of bright orange or cream-coloured chest fur that stands out against the rest of its jet-black coat.

It is the smallest bear – about the size of a large dog – and is listed as a vulnerable species due to deforestation and the global wildlife trade.

Other Chinese zoos have previously been accused of stocking their enclosures with animals that are not what they seem – including a hairy dog passed off as an "African lion" and a pool filled with penguin-shaped balloons. (AFP)