"An attempt by Russia and Iran to prop up Assad and try to pacify the population is just going to get them stuck in a quagmire and it won't work," Obama told a White House news conference.
He said Russia is also failing to distinguish between Islamic State and more moderate insurgents in Syria.
"From their perspective, they're all terrorists," Obama said. "And that's a recipe for disaster."
Russia bombed Syria on Friday for a third straight day, mainly hitting areas held by rival insurgent groups rather than the Islamic State fighters it has said it was targeting, and drawing an increasingly angry response from the West.
Russian President Vladimir Putin's decision to launch strikes on Syria marks a dramatic escalation of foreign involvement in a more than four-year-old civil war in which every major country in the region has a stake.
