Police in Moscow have reportedly arrested a woman who was waving the severed head of a small child outside a subway station in the Russian capital.

Videos posted on Russian news websites show the woman dressed all in black and shouting "I am a terrorist," although most of what she says is incomprehensible.

The Investigative Committee released a statement saying a woman was arrested on suspicion of killing a child aged three or four in an apartment near the metro station in north-western Moscow, and then setting the apartment on fire.

The statement said the woman was believed to be the nanny.

The investigators said the woman appeared to be mentally unstable.

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