An armed student entered a Moscow school on Monday and took more than 20 teenagers hostage in an incident that also claimed the lives of a policeman and a teacher, security officials said.

The hostage-taker was one of the students of school number 263 on the northern outskirts of Moscow, police officials said.

A policeman and a biology teacher were killed in the hostage crisis, which sparked fresh security fears in Russia ahead of the start of the Winter Olympic Games in Sochi this week.

He had held more than 20 teenage biology class students and the teacher hostage for about 30 minutes before the police moved in, news reports said.

It was not immediately how the teacher and the policeman died.

School official Avitara Sharafetdinova told Vesti 24 state television channel that all children and teachers had been evacuated from the school after the incident.

"All the children have been evacuated," Sharafetdinova said in televised remarks. "There is no one there anymore."

Vesti 24 said the student had made no demands and conducted no negotiation with the police before being "neutralised."

"He has been neutralised," Russian interior ministry spokesman Andrei Pilipchuk told Vesti 24.

A later report said the hostage-taker was detained.

Live footage from the scene showed a group of children running from the school and an emergencies ministry helicopter hovering overhead.