A woman has been detained outside a Moscow metro station and charged with murder after she walked around with a young child's severed head reportedly shouting "Allahu Akbar." A graphic video posted to social media appears to show the suspect.

Moscow investigators said the murdered child was three or four years old and that the nanny had killed her in the family apartment in the Russian capital before setting fire to the premises and fleeing.

Officials 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.

Local media reported that the woman — who was wearing a burka — remained outside the station for an hour before being detained by police.

Lifenews.ru, an online news portal with close contacts to the police, 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 she had heard the woman screaming "Allahu Akbar" (God is Great). "I was on my way to the metro station from home," Polina Nikolskaya, the reporter, told Reuters.

"She was standing near the metro entrance and caught my attention because she was screaming Allahu Akbar. I saw that she had a bloodied head in her arms, but I thought it was not real. People in the crowd said it was real."

In other footage of the scene, the woman can be heard shouting about the end of the world while proclaiming herself a terrorist.

The Russian Interior Ministry has also confirmed that the suspect was carrying a real human head, according to the Telegraph, who also reported that police are not currently treating the incident as terrorism.