Travis Tygart said Armstrong had been pushed into saying he would co-operate with a new inquiry into doping in cycling because of legal proceedings in the U.S.
"He is going for a deposition in the United States in November where he is going to go under oath in a lawsuit and have to answer questions and I think that he is now being forced essentially through that process (to come clean) and that he is trying to gain an advantage," Tygart told Reuters.
"It's a little late but we are still hopeful he will come and answer everything we have to ask him under oath but until he decides to do that, it is entirely premature to determine or speculate on any sort of reduction (of his life ban)."
Armstrong, who was stripped of his seven Tour de France titles for doping, told the BBC this week that he would testify with "100 percent transparency and honesty" if asked to appear at an inquiry.
Tygart, attending the World Conference on Doping in Sport, said the American had been give chances to tell his side of the story but had declined.
"We invited him to come in June 2012 at the same time as we invited other athletes guilty of doping. He was the only one of the 11 that refused our offer," he said.
"We attempted to meet again in December and in January and February this year and so far he's refused to come in and be truthful and answer all the questions under oath just like all the other athletes have done, so at this point we are going forward.
"We are hopeful that we'll get to the bottom of a deep culture of doping that took over the sport and give clean athletes final hope that they can compete successfully without having to use dangerous performance-enhancing drugs."
Cycling's governing body said only the USADA could consider any reduction of Armstrong's life ban from the sport.
"USADA is the body that has sanctioned Armstrong and those (sanctions) have been accepted by the UCI and WADA (World Anti-Doping Agency)," Brian Cookson, president of the International Cycling Union (UCI) told Reuters.
Cookson, who said the UCI and WADA had agreed the terms of a doping inquiry, added: "(The USADA) may well want to look at it if Mr. Armstrong is prepared to contribute to the commission and provide further information.
"I think one of things that really interests me is allegations he seems to have made that apparently there has been collusion in the past with the UCI. If that is true, those are very serious allegations and I'd like to see some evidence of that."
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