ISTANBUL — An Islamist rebel group said Thursday that it had taken control of Hama, Syria’s fourth-largest city, dealing another significant blow to President Bashar al-Assad less than a week after his forces ceded Aleppo in the face of a breakneck rebel advance.

       The Islamist group, Hayat Tahrir al-Sham, or HTS, which is leading the offensive, released a series of short statements Thursday afternoon, saying it had entered neighborhoods in Hama after days of fighting on its outskirts and freed inmates from the city’s central prison. Around the same time, the Syrian army announced it had withdrawn.