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PARIS -- President Obama laid out his case once again for his Syria strategy built around a combination of military air power to degrade the Islamic State and diplomatic pressure to push out Syrian President Bashar al-Assad.

Obama was in Paris on Tuesday for a major summit on climate change in which he voiced optimism that the world was moving toward a solution to the global problem. "The main message I got is that I actually think we are going to solve this thing," he said of the climate initiative. When it came to fighting the Islamic State in Syria and Iraq, the president did not muster the same optimism.

Obama has been criticized -- especially in the wake of the Islamic State's attacks on Paris -- for not doing more militarily to defeat the extremist group. His defense of his strategy in Paris showed the complexity of the task he faces. He expressed confidence that Russian President Vladimir Putin could alter his strategy in Syria and ultimately support a political solution there that includes the exit of Assad.

"And I think Mr. Putin understands that, with Afghanistan fresh in the memory, for him to simply get bogged down in an inconclusive and paralyzing civil conflict is not the outcome that he’s looking for," he said, noting Russia has lost a commercial passenger jet, a fighter jet and Russian personnel since it entered the conflict.

But he warned that he did not expect Putin, whose forces have been attacking moderate opposition fighters rather than the Islamic State, to change quickly. "I don't expect that you are going to see a 180 turn on their strategy over the next several weeks," he said of Russia's strategy. "They have invested for years now in keeping Assad in power. Their presence there is predicated on propping him up."

Obama said he was not under "any illusions" that Russia would bomb only Islamic State targets and spare the moderate opposition forces that the United States and its allies are backing. But he expressed hope that a political process, led by Secretary of State John F. Kerry, could help stitch together local cease-fires between Assad and the moderate opposition.

"That may mean then that certain opposition groups no longer find themselves subject to Syrian or Russian bombing," Obama said. "They are then in a conversation about politics."

Obama also talked about the imperative of Turkey sealing off its border to Islamic State fighters and the need to choke off the radical group's finances. Here, once again, the president acknowledged complications. Turkey has kept its borders open with Syria to protect refugees in "real need," Obama said. The Turks were also dealing with "enormous strains" on their economy because of the refugee crisis.

Obama vowed to step up efforts alongside the Turks to prevent fighters from making their way from the West to Syria, where they can become battle-hardened terrorists and lead attacks such as the ones that hit Paris. "We have to cut off their source of new fighters," Obama said.

Obama also addressed the Planned Parenthood shooting Friday in Colorado Springs, Colo., striking a familiar theme on the need for gun control.  "We are rightly determined to prevent terrorists attacks wherever they occur," he said. "We devote enormous resources and rightfully so ... and yet we have the power to do more to prevent what is just a regular process of gun homicide."

Eilperin and Jaffe reported from Washington.