Russian investigators said today they had arrested a nanny and charged her with murdering a young child in her care after local media broadcast footage of a woman brandishing a severed head near a busy Moscow metro station.

Moscow investigators said in a statement the unnamed nanny was from Central Asia and that she was undergoing psychiatric testing to see if she was mentally sound and understood the significance of the crime they said she had committed.

Lifenews.ru, an online news portal with close contacts to the police, published video footage of a woman it identified as the nanny. Clad in black, she was seen wandering around the street holding up a child's severed head high in the air.

The portal said a policeman had approached the woman to check her documents and that she had responded by removing the child's head from a bag and begun shouting that she had killed the infant.

One reporter, from the RBC daily, said the woman had also been heard screaming "Allahu Akbar" (God is Great).

Investigators said the murdered child was three or four years old and that the nanny had killed her in the family's Moscow flat before setting fire to the premises and fleeing.

The motive for the crime was unknown, they said.