BRUSSELS — Russia’s campaign of airstrikes against opponents of President Bashar al-Assad of Syria has stabilized Mr. Assad’s government, America’s top general said Wednesday. That has probably given Mr. Assad a stronger hand to play next week, when negotiations toward a political solution to the conflict will begin in Geneva, American officials said.

Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said Russia’s entry into the crowded battlefield had not changed how the American military was proceeding in Syria. He said the American-led coalition battling the Islamic State there and in Iraq had made significant gains, retaking an important dam on the Euphrates River and a large stretch of territory north of Raqqa, Syria, where the militant group has its stronghold.

The campaign to isolate Raqqa from other Islamic State-controlled territory — and in particular, from the Iraqi city of Mosul, whose fall to the militants in 2014 seized international attention — is well underway, General Dunford said. The main highway between Raqqa and Mosul has been cut, he said, and coalition troops are working to block smaller roads that link the two cities. That will impede supplies from reaching Islamic State fighters in Mosul, the objective of an expected Iraqi offensive.

While “there’s still freedom of movement between Mosul and Raqqa, current operations are designed to cut them,” General Dunford told reporters traveling with him to a meeting of the chiefs of staff of the militaries of NATO countries.