A nanny beheaded a young girl in her care and brandished her severed head at a metro station in Moscow before being arrested at the scene.

The veiled woman, dressed in all black, shouted on Monday that she was "a terrorist", but police said they suspect she is mentally unsound.

"According to preliminary information, the child's nanny - a native of one of the Central Asian countries, born in 1977 - waited until the parents left the apartment with their elder child and - guided by unknown motives - killed the little one, set the apartment on fire and left the scene," the police investigative committee said in a statement.

Investigators said the woman was arrested and a criminal investigation opened.

They said that the suspect would undergo a psychiatric examination to establish whether she "understands the meaning of her actions and behaviour".

Polina Nikolskaya, a reporter at the RBC daily newspaper, saw the women on the street. 

"She was standing near the metro entrance and caught my attention because she was screaming 'Allahu Akbar'," she told the Reuters news agency. 

"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."

LifeNews, a news service known for its close ties to law enforcement agencies, said that when police approached the woman she responded by taking the head out of her bag and started yelling that she had killed a child.

Source: Agencies