MOSCOW – President Dmitry Medvedev sacked Russia's most powerful spymaster on Friday in a move that underscores strained ties with some of the military top brass over a Kremlin-backed reform of the armed forces.

The Kremlin said Medvedev had signed a decree to dismiss General Valentin Korabelnikov, who has directed Russia's military intelligence service since 1997.

The dismissal of such a respected 63-year-old spymaster is one of Medvedev's biggest sackings since he replaced former KGB spy Vladimir Putin as president in May 2008.

The Kremlin, which gave no reason for sacking, said Alexander Shlyakhturov, a first deputy in the military intelligence service, was appointed as his replacement.

Korabelnikov had criticized reforms which the Kremlin says aim to turn Russia's outdated army into a mobile fighting force. The number of generals will be slashed and the headcount of the armed forces will be cut to 1 million from more than 1.1 million.

This is a blow, a blow at our Russian forces and the security of our nation, Viktor Ilyukhin, a deputy for the opposition Communist party who used to serve as deputy head of parliament's national security committee, said of the sacking.

He was a born intelligence chief, a man who brilliantly knows the situation in the world and one of the best experts in the situation in Russia's army, Ilyukhin told Reuters.

The cost of insuring Russia's sovereign debt in the credit default swaps market was unchanged on the day, with the firing seen as not enough to significantly unsettle markets.

Russia's military intelligence service, known as GRU, is the country's biggest spy agency with agents spread across the globe. It also has thousands of special forces troops inside Russia, some of whom were deployed in Georgia last year.

The service, created in 1918 under revolutionary leader Leon Trotsky, is controlled by the military general staff.

From the beginning, Bolshevik leader Vladimir Lenin insisted on its independence from other Soviet security services and since then the top-secret body has been widely seen as a staunch rival of other spy agencies.

It has no website or spokespeople unlike Russia's smaller Foreign Intelligence Service (SVR) and the FSB domestic spy agency, both of which emerged from the KGB after the collapse of the Soviet Union in 1991.

Russian media, quoting Defense Ministry sources, said Korabelnikov had clashed with Defense Minister Anatoly Serdyukov, appointed by former President Putin, over plans to disband or reorder special forces units under GRU command.