This has given them control of much of the city of Aleppo, Syria's largest, and one provincial capital, Raqqa.

NPR: Syria Opposition Pushes To Form Interim Government

Reports say clashes were continuing in parts of Raqqa, with more troops sent.

BBC: Syria crisis: Raqqa governor held by rebels 'as city falls'

Only in the last week did the Syrian regime intensify strikes on Raqqa, a city in Syria's interior that is considered ISIS's headquarters.

CNN: ISIS right outside Baghdad

Syrian warplanes have bombed the northern city of Raqqa, hours after reports said rebels had overrun it, activists and residents say.

BBC: Syria crisis: Warplanes 'bomb Raqqa after rebel gains'

Of those, 18 were in Aleppo and four were in Raqqa.

CNN: Execution video surfaces in Syria

Raqqa city is near Syria's northern border with Turkey and residents say it has become home to thousands of people forced to flee their homes in other war-torn parts of Syria.

BBC: Syria: Fierce clashes in provincial capital Raqqa