LONDON (AP) — London police are investigating a small explosion at a London Underground station.

Police said they are not aware of any serious injuries in what the force called a "minor explosion" at Southgate tube station in northwest London on Tuesday evening.

British Transport Police said the Southgate Tube station was closed while police investigate a suspicious package. Police cordons were put in place and the public was asked to keep away.

The Metropolitan Police said officers remained on the scene and were investigating.

Transport police say a "small number" of people were treated at the scene. The nature of the injuries was not immediately clear. There were no immediate reports of anyone needing hospital treatment.

Police responded after people ran from the station and said there had been an explosion.

Police said no cause has yet been established and the nature of the incident is unclear.

Britain's official terrorist threat level has been set at "severe," indicating that intelligence services feel an attack is highly likely.