Russia has replaced the head of its GRU military intelligence organisation, who presided over major reforms of the secretive body, a defence ministry spokesman said on Monday.

"Major General Igor Sergun has been appointed the head of the GRU through a Kremlin decree," a defence ministry spokesman told AFP.

Sergun replaces Alexander Shlyakhturov, who had spearheaded a shake-up of the service since his appointment in 2009. The defence ministry spokesman did not provide further details.

The state RIA Novosti news agency said, citing a ministry spokesman, that Shlyakhturov had reached retirement age. No other reason was given for the move.

The GRU (Main Intelligence Directorate) traces its history back to 1918 in the early months of the Bolshevik revolution, and unlike other Russian secret services did not change its name after the collapse of the Soviet Union.

It reputedly runs vastly more agents abroad than the KGB's successor, the Foreign Intelligence Service (SVR), and is also active in space through a network of military intelligence satellites.