President Barack Obama held firm that Syrian President Bashar al-Assad must be removed from office, saying Thursday that he doesn’t believe the civil war in Syria will end while the dictator remains in power.

After a meeting in Manila with Canadian Prime Minister Justin Trudeau, the U.S. president said his assertion that Mr. Assad must go was a reflection of reality on the ground in Syria.

“The reason is not simply because of my opinion of him. It is because it is unimaginable that you can stop the civil war here when the overwhelming majority of people in Syria consider him to be a brutal, murderous dictator,” Mr. Obama said. “He cannot regain legitimacy. And if in fact he is still in power, regardless of what outside powers do, there is still going to be large portions of the population that are fighting.”

Questions about Mr. Assad’s future have become a stumbling block as French President François Hollande works to assemble a “grand coalition” led by his country, the U.S. and Russia working together to combat Islamic State militants.

Mr. Hollande has long argued for Mr. Assad’s ouster, saying that the Syrian president’s atrocities led to the creation of Islamic State. Moscow has supported Mr. Assad, suggesting that the Syrian state would collapse if he were removed from power.