Fighters from the extremist Islamic State group on Thursday overran part of an army base in northern Syria, which has been under the militants’ siege for months, in ferocious battles that killed or wounded dozens on both sides, activists said.

The battle over the base is the latest in the Islamic State’s push to capture as much of Syrian territory as it can. Since June, the group has seized a huge chunk of territory straddling the Iraq-Syria border, where they have declared a self-styled caliphate.

The base lies in Raqqa province, where much of the territory fell to the Syrian opposition last year. Earlier this year, the Islamic State, which sided with the rebels at the start of the Syrian conflict three years ago, captured much of Raqqa and has tried to capture the base several times. […]