Dozens of fire engines and ambulances swarmed around the Federal Security Service (FSB) building on Lubyanka Square after the fire was reported at 0215 local time (2315 GMT).

Firefighters put out the blaze in the building, at the heart of the Russian capital, some 90 minutes later, according to the Itar-Tass news agency.

A police spokesman said the blaze began in a second floor office of the building before spreading to another office and the corridor. It then spread through ventilation shafts to parts of the third and fourth floors.

The cause of the fire was not immediately clear but officials suspect either an electrical problem or a "violation of fire safety regulations", Tass said.

Arson is so far thought unlikely, but an investigation will be carried out by fire service specialists and the FSB, according to Tass.

Injured

Tass said 10 people had been evacuated down ladders and that four FSB workers had been injured - one of them seriously.

The Lubyanka building has been the headquarters of the Soviet and then the Russian security services for around 80 years.