REYHANLI, Turkey — Anti-government media activists and rebel commanders gave a mixed assessment of U.S.-led airstrikes in northern Syria on Tuesday, saying that some of the Islamic State encampments hit had been evacuated and one building that was struck had been filled with displaced civilians, even as at least one major Islamic State base was seriously damaged and many fighters were killed. But the greatest damage, they said, may be to the Free Syrian Army, the moderate rebel faction that enjoyed U.S. support for years.