Officials said the explosives used in the blasts were produced in the Chechen city of Urus-Martan, and a cache of the same type of explosives was discovered after the city fell to Russian troops fighting in the breakaway republic.

FSB official Nikolai Sapozhkov said that five of the six accused had been charged in absentia.

The sixth suspect, identified as Ruslan Magayayev, is being held in Moscow's Lefortovo prison, Mr Sapozhkov said.

No details were given about the suspects' nationalities.

Attacks averted

Another senior FSB official, Alexander Shagako, told the news conference that the agency had prevented at least six other major terrorist attacks in Moscow.

Mr Sapozhkov said the charges related to two of the blasts in Moscow last September. He said the accused had transported the explosives, disguised as sugar, from Chechnya to Moscow by road.

"There are no people in the Russian secret services who would be capable of such crime against their own people," Mr Putin, a 15-year KGB veteran, told the Kommersant newspaper.

"The very allegation is immoral."