Security forces have executed Free Syrian Army colonel Hussein Harmush, a founder of the rebel group made up of soldiers who defected, the Syrian League for Human Rights said on Monday.
"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 the regime's deadly crackdown on protesters while speaking to AFP in the Turkish village of Guvecci.
Months later, 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.