An initial investigation suggested a gas leak was to blame, but other possible causes were also being considered, said Mr Marat Prokopenkov of the Emergency Situations Ministry's branch in southern Russia.

The blast destroyed the three-story brick headquarters of the regional bailiffs' service in Ingushetia's main city, Nazran.

Among the 18 people injured was a pregnant woman, said spokesman Svetlana Gribakova of the regional branch of Russia's Investigative Committee. She said two victims were in critical condition, but gave no further details.

Attacks on law enforcement officials are common in Ingushetia.

The province borders war-scarred Chechnya and is part of Russia's restive North Caucasus. Near-daily outbreaks of violence stem from Islamic militancy, business and personal rivalries and deep-seated anger at government oppression and heavy-handed police tactics. -- AP