BAGHDAD — The Nusra Front, a Syrian rebel group affiliated with Al Qaeda, has been expanding its control in the northern province of Idlib, seizing territory from two Western-supported rebel organizations and potentially threatening a critical border crossing with Turkey, according to rebels and monitoring groups.

Groups in Idlib have been a focus of the Obama administration’s plan to train and equip some Syrian rebels to fight Islamic State, the jihadist group that has occupied territory further east in Syria and Iraq. And though the province represents just a small part of the sprawling conflict in Syria, it has been an important center of international attempts to organize and supply the resistance to President Bashar al-Assad’s government.

Two groups that the Nusra Front has seized bases from in recent days, the Syrian Revolutionaries Front and the Hazm Movement, are considered moderates and have received limited arms support from the West. Despite that, they have been unable to hold their ground against the extremists in this latest outbreak of rebel infighting, commanders say.

The trouble began last week when the Nusra Front began moving against the Syrian Revolutionaries Front, headed by Jamal Maarouf, a former construction worker-turned rebel leader who has been widely accused by other rebels of war profiteering.