Hajj gave an exclusive interview to The Anadolu Agency on Monday in the southern Turkish province of Gaziantep saying, "Mistura is implementing the regime agenda and we know the places which he visited when he was in Damascus."
Staffan de Mistura was appointed in 2014 as the new special envoy tasked with seeking a peaceful resolution of the conflict in Syria by United Nations Secretary-General Ban Ki-moon.
“Mistura is working for Assad. His plans and opinions are the same as the regime’s demands. He has lost his credibility by expressing his opinions and he was supposed to be unbiased," he said.
"He is doing what the regime wants, but we are being persecuted by the regime," Hajj added.
Hajj claimed that the regime is still using chemical weapons against civilians and said that Mistura should put his efforts into showing the plight of Syrian civilians.
Syria has been gripped by almost constant fighting since the Assad regime responded to anti-government protests in March 2011 with a violent crackdown that sparked a conflict spiraling into a civil war.
As many as 3.8 million Syrian refugees are registered in neighboring Turkey, Jordan, Lebanon and Iraq, according to the UN.