Hong Kong (AFP) - Hong Kong police say they are hunting a girl aged between 12 and 14 years old over the theft of a diamond necklace worth more than HK$36 million (US$4.6 million) from a luxury jewellery store.

Police say she helped carry out the audacious theft with two women and a man, all aged between 30 and 40.

The adults "pretended to be customers choosing items from the shop" police said, while the girl stole the necklace.

The gang of four then left the store before a staff member realised it was missing.

The necklace was 100-carat gold embedded with more than 30 diamonds, the South China Morning Post said, and police confirmed it was worth HK$36 million.

The young girl managed to steal a key from a drawer to open a cabinet while staff were distracted, the SCMP said, citing an unnamed police source.

"The three adults posed as big spenders and demanded employees show them jewellery in an apparent move to divert staff attention," the source said.

Local media reported that the theft was from the Emperor Jewellery store in the Tsim Sha Tsui shopping district, but the store would not confirm it had been targeted when contacted by AFP.

The girl was caught on CCTV as she left the mall, the SCMP said, showing a picture of a girl dressed in black trousers, a blue shirt and a grey hooded top.

Police described her as "slimly built".

The gang spent around half an hour in the shop and staff did not notice the necklace was missing until later in the day, the SCMP added.