The theft took place at an upmarket outlet in Marina Bay Sands resort, the world's second most expensive building at USD 5.5 billion. Police are investigating the case, The Straits Times said.
Report said a man and a woman along with the girl entered Bulgari store situated within the resort premises on the Christmas night and behaved like a family.
As the couple engaged the sales staff, the girl allegedly swiped the ornament from an unsecured display case.
The exact value of the stolen ornament was not immediately known.
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Police have issued letters to shops in the report alerting them to the high-value jewellery theft.
The case was said to be similar to last Friday's theft in Hong Kong where a girl, believed to be 12-14 years-old, unlocked a case in the Emperor Jewellery shop in the 188s Heritage shopping mall in Tsim sha Tsui area.
The modus operandi is said to be similar to an incident in Kuala Lumpur shopping district on Jan 16 last year, when three women with a girl aged around 10 years made off with SGD 1.9 million (USD 1.5 million) yellow diamond ring.