The Free Syrian Army (FSA), composed of defectors from the government armed forces, killed seven of their former employers in Idlib province Tuesday.

A dissident Syrian soldier shows his ID card alongside five army officers and other conscripts as they defect to the rebel Free Syrian army. (YouTube/AFP/Getty Images)

First formed in June, the FSA has swelled in size in recent weeks, encouraging defections by ambushing patrols, shooting commanders, and then convincing the soldiers to switch sides. While their numbers are not known, they are led by Colonel Riyad al-Asad, a former Colonel in the Syrian Air Force who defected in June. Since then, the army has increasingly employed guerilla-style tactics to fight government forces, carrying out hit-and-run attacks or planting bombs on buses.

They have also promised President Bashar al-Assad: “You will find us everywhere at all times, and you will see that which you do not expect, until we re-establish the rights and freedom of our people.”

Footage has emerged of the Free Syrian Army that gives more insight into who makes up their ranks — both flag-wielding civilians and uniformed military defectors:

The Syrian Free Army is being viewed as the strongest challenge yet to Assad’s regime in the the nine-month-old uprising. But human rights activists warn that the army’s violence could also make it more difficult for the West to give diplomatic support to the opposition.

The United Nations this week put the death toll in Syria at 5,000 and warned that the country could be sliding further into civil war.