Fighters from the Nusra Front, al Qaida’s official affiliate in Syria, on Tuesday seized three strategic towns on the border with Turkey in a major blow to U.S.-backed moderate rebels.

Nusra’s seizure of Izmarin, Salkin and Harem in Syria’s Idlib province came only four days after the group seized Darkoush, another border town, from the Syrian Revolutionary Front, a moderate group that is part of the Western-backed Free Syrian Army.

The move appears to be part of an effort by Nusra to add new area to what it has called its emirate in northern Syria, a designation the al Qaida-affiliate made in response to the declaration last month of an Islamic caliphate in Iraq and Syria by its arch-rival, the Islamic State. Since the Islamic State’s designation of its caliphate, Nusra has lost territory to the Islamic State. Many of Nusra’s best known commanders also have defected to the Islamic State. […]