Game over for Syrian opposition in Aleppo?

Editor's Note: Charles Lister is a visiting fellow at the Brookings Doha Center in Qatar, where his work focuses particularly on terrorism and insurgency in Syria, Iraq and Lebanon. He is writing a book on the jihadist insurgency in Syria. Follow him on Twitter@Charles_Lister. The opinions expressed in this commentary are solely those of the author.

(CNN) - The fate of Syria's Western-backed opposition hangs on a knife edge in the northern part of the war-torn country -- and with radical Sunni militants and regime forces closing in on them from all sides, time may be running out.

At least six villages north of Syria's largest city of Aleppo fell Wednesday to militants from ISIS, according to AFP. The jihadist group has seized large swathes of land in Iraq and consolidated control over considerable territory in northeastern Syria in the past year.

ISIS fighters are now just 30 miles from the rebel-controlled northern suburbs of Aleppo and within striking distance of key opposition positions leading to the Turkish border.

The situation for the opposition may be even worse inside Aleppo city, where forces loyal to President Bashar al-Assad are gaining ground after a brutal months-long campaign against opposition forces.

With the radical Sunni fighters bearing down on them from the north, and troops loyal to President Assad retaking Aleppo neighborhood by neighborhood from the south and west, Syria's beleaguered rebels are facing an existential threat.

Since November 2013, the Syrian government has executed a concerted offensive on opposition-controlled areas of Aleppo city. Intensive and horrifically destructive barrel bombs have flattened the urban environment in which opposition insurgents had thrived, and forced thousands of civilians to flee.

The military has followed air bombardment with methodical but effective ground incursions that, over time, have enabled it to re-capture territory and force a rebel retreat to the city's northern districts. As such, the opposition is now in its weakest position in Aleppo city since mid-2012.

ISIS burst onto the scene in Syria in April 2013 and by December it had successfully established an expansive territorial presence across northern Syria, including in Aleppo governorate. But a sustained rebel offensive in January of this year forced ISIS to withdraw from the northwestern governorates of Latakia and Idlib and much of Aleppo, with the exception of three main towns in Aleppo's northeast: Al-Bab, Manbij and Jarablus.

The rebels in Aleppo fighting to topple Assad and beat back ISIS' advance have long been a melting pot of different groups, including various Free Syrian Army (FSA) units, the al Qaeda affiliate Jabhat al-Nusra, and the Islamic Front (IF). While core moderate FSA-linked factions benefitted from an influx of Western support in late 2013 to facilitate their anti-ISIS offensive in January, this support has since dwindled. A principal Aleppo-based beneficiary of this support, Jaish al-Mujahideen, has since become a shadow of its former self.

The U.S. has leaned heavily on Gulf states to reduce their support of Islamist and Salafist groups in Syria, which has damaged the IF's capacity to operate as a unified and effective coalition. In Aleppo specifically, IF's main group, Liwa al-Tawhid, has also suffered from debilitating internal divisions and defections.