PARIS — President Barack Obama expressed optimism today that Russia will ultimately come around on the need for Syrian President Bashar Assad to leave power to end his country’s long civil war, but warned the turnaround would not come overnight.

Discussing the Syria crisis with world leaders in Paris, Obama also urged Turkey and Russia to set aside recent tensions that have undermined his efforts to strengthen the U.S.-led coalition fighting defeating the Islamic State group. Instead, he asked the two countries to focus on IS as a common enemy and on reaching a political solution for Syria.

Obama’s remarks on the sidelines of global climate talks came as the U.S. continues to press Russia to focus its airstrikes in Syria against IS, rather than on U.S.-backed rebel groups fighting Assad. Obama said it was possible Russia would undergo a “shift in calculations” and back away from its support for Assad.

“I don’t expect that you’re going to see a 180-degree turn on their strategy over the next several weeks,” Obama said.