“An air force intelligence unit last week carried out a sentence to shoot dead officer Hussein Harmush,” the non-governmental group said in a statement.
The report could not be immediately confirmed.
In June, Harmush became the first Syrian military officer to publicly declare his opposition to President Bashar al-Assad regime’s deadly crackdown on protesters while speaking to AFP in the Turkish village of Guvecci.
Harmush left Syria seeking exile in Turkey, where he established the Brigade of Free Officers, a group of dozens of army deserters later absorbed into the Free Syrian Army headed by Riyadh al-Asaad.
In September, his “confessions” were aired on Syrian national television after his return home in unclear circumstances. Many refugees are convinced he was kidnapped by Syrian agents from a refugee camp.
The Cairo-based Syrian League for Human Rights urged the “international community to intervene to protect detainees in Syrian jails from the physical liquidations they face on a daily basis.”