Japan's panda-obsessed public got their latest fix of cute Tuesday as a Tokyo zoo released footage of its 20-day-old helpless cub, who has ballooned in size since its birth last month.

The tiny panda, still without a name, was the first to be born at Tokyo's Ueno Zoo in five years to the delight of animal lovers and local businesses, sparking 'panda fever' in the capital.

Distinctively pink and hairless, the animals typically weigh around 100 grams (three-and-a-half ounces) at birth -- small enough to fit in the palm of a human hand.

It can be difficult to determine their sex but the zoo confirmed last month that the newborn was a girl based on pictures sent to a research centre in the shy animal's native China.

At two days old, she weighed just 147 grammes (5.2 oz) and measured 14.3 centimetres (5.6 inches) in length, but zoo officials have since said she is growing well.

Footage shows the cub resting comfortably on a blanket Sunday as caretakers conducted a regular health check, with the cub now weighing in at 607.9 grammes and measuring 23.4 centimetres.

The tiny baby also looks increasingly more panda-like, with the trademark black colour around her eyes, ears and feet becoming clearer.

Her eyes remain closed as it usually takes about 40 days before panda cubs start seeing things, the zoo said.

In another shot, mum Shin Shin, who just turned 12 on Monday, is seen tenderly holding the baby in her right arm.

Considered an endangered species, it is estimated that fewer than 2,000 giant pandas remain in the wild, in three provinces in south-central China.

Shin Shin, who mated with male Ri Ri in February, had given birth to another cub in 2012 -- the first time at the zoo in 24 years -- but the baby died from pneumonia just six days later.