ISIS, an Al-Qaeda offshoot which has seized swathes of territory and declared an Islamic caliphate in areas it controls in Iraq and Syria, has controlled Raqqa’s provincial capital since the start of the year.
Last month the group killed at least 50 Syrian government forces as they took over parts of another base in the area.
On Thursday, a monitoring group and ISIS supporters said the militants had taken full control of the 93rd Brigade military base after an attack that opened with multiple suicide car bomb attacks.
“We announce…the total liberation of the 93rd Brigade,” said a Twitter feed which regularly publishes news from ISIS in Raqqa province.
The Britain-based Syrian Observatory for Human Rights said at least 27 pro-government fighters were killed after three Islamic State fighters blew themselves up in car bombs at the gates and around the base and in the ensuing clashes.
At least 11 ISIS fighters were killed, added the group, which monitors violence in Syria through a network of sources. It said “dozens” more were wounded.
ISIS has made rapid advances in Syria since its forces along with allied Sunni militants took control of Mosul, northern Iraq’s largest city, on June 10.
The group was once an Al-Qaeda-affiliated branch in Iraq, but formally split with Al-Qaeda this year after tensions rose over its expansion into Syria. Al Qaeda’s affiliate in Syria, the Nusra Front, has occasionally clashed with the Islamic State since then, but also cooperated in some areas.
The Observatory estimates that ISIS controls about 35 percent of Syrian territory—although much of that is desert.
The government has meanwhile consolidated its grip on the country’s more densely populated central areas, including a corridor stretching from the capital Damascus to the Mediterranean coast in the west.
More than 170,000 people have been killed in Syria’s civil war, which pits overwhelmingly Sunni Muslim rebels against President Bashar Al-Assad, a member of the Shi’ite-derived Alawite minority, backed by Shi’ite militias from Iraq and Lebanon.