Putin called Litvinenko's death a "tragedy," but said he had seen no definitive proof as yet that it was a "violent death."
Litvinenko died in a London hospital overnight, three weeks after being poisoned.
In a deathbed statement read out by friends, Litvinenko accused Putin of responsibility for his death.
The Kremlin has repeatedly denied that the Russian government was in any way involved.
British authorities today said Litvinenko was poisoned by radiation. Experts said today they had found a highly toxic isotope -- probably polonium-210 -- in Litvinenko's body.
(compiled from agency reports)