Russia's involvement in Syrian war has led to significant changes, including "shrinking" of the Islamic State, President Bashar al-Assad said.
Russia launched air strikes against IS in late September in support of Assad, Moscow's ally, BBC reported.
Assad said the US air strikes had not slowed the IS group in Syria, but Russian bombing was doing so.
He also condemned Turkey's shooting down of a Russian warplane on November 24. Ankara said Su-24 "violated" Turkish airspace, but Moscow has denied the claim.
Assad said the incident showed that Turkish President Recep Tayyip Erdogan had "lost his nerve" because Russia's involvement had altered what was at stake.
"The failure of Erdogan in Syria, the failure of his terrorist groups, means his political demise," he said.
The US last month said most of the air strikes were against "moderate Syrian opposition forces" opposed to Assad.
Assad said that since the US air strikes began in September last year, IS "has expanded and the recruiting from around the world has increased".
"Since the participation of Russia in the same fight... IS has been shrinking and al-Nusra Front of course, and the other terrorist groups. The facts are telling," Assad said.
However, in September, before Russia began strikes, the US Defense Department said IS had lost up to 10 percent of its territory in Syria in a year.
