The NTC official, Abdel Majid Mlegta, had told Reuters earlier that Gaddafi was captured and wounded in both legs at dawn on Thursday as he tried to flee in a convoy which NATO warplanes had attacked.
"He was also hit in his head," said Mlegta. "There was a lot of firing against his group and he died."
Asked if there was photographic evidence to prove that Gaddafi was dead, Mlegta said: "We have the footage but it is not available now."
There was no independent confirmation of his remarks.
Mlegta reported Gaddafi's death after Libyan interim government fighters took Sirte on Thursday, extinguishing the last significant resistance by forces loyal to the former leader and ending a two-month siege.
NATO warplanes struck the convoy and hit four cars as it headed west, Mlegta said, adding that the head of Gaddafi's armed forces Abu Bakr Younus Jabr had been killed during the attack.
Ahmed Ibrahim, a cousin and adviser of Gaddafi, was captured along with former government spokesman Moussa Ibrahim, he added.
In the capital Tripoli, sounds of gun shots were heard and people cheered in the street: "God is Great, God is Great, Gaddafi has been captured."