Fort Meade (United States) (AFP) - US President Barack Obama said Friday that Russia's decision to send military advisors and equipment to Syria extended a strategy "doomed to failure," and could kill off already stagnant peace efforts.

Obama said that Russia's decision to ramp up its military presence in war-torn Syria represented a "doubling-down" of Moscow's support for Syrian President Bashar al-Assad.

US officials say Russia has sent ships, armored personnel carriers and naval infantry to the country in recent weeks, a move which Obama said showed Assad was worried his grip on power is slipping.

"The strategy that they are pursuing right now, doubling down on Assad, I think is a big mistake," Obama said at an event marking the 14th anniversary of the 9/11 attacks.

"We are going to be engaging Russia to let them know that you can't continue to double down on a strategy that is doomed to failure."

He said Russia -- which has a large and restive Muslim population in the North Caucasus -- faced more of a threat from the Islamic State group than the United States does.

"The Russians are going to have to start getting a little smarter than they have been," he said.

Obama also said the Russian move would not have an impact on the US "core mission" to fight the Islamic State group in Syria, but added "it could prevent us from arriving to a political settlement."

The United Nations and countries with influence in Syria have been unable to agree on a way to end the four-year-old conflict that has killed almost 200,000 people and prompted the most serious refugee crisis since World War II.