WADA director general David Howman told The Associated Press that Armstrong's interview with Winfrey is "hardly the same as giving evidence to a relevant authority" that deals with doping rules and sanctions.
"He's got to follow a certain course," Howman said. "That is not talking to a talk show host."
Armstrong was stripped of his seven Tour de France titles and banned for life from Olympic sports last year following a U.S. Anti-Doping Agency report that portrayed him as a longtime performance-enhancing drug user. After years of denials, he confessed to doping during an interview with Winfrey taped Monday.
Armstrong has been in conversations with USADA about a possible confession to authorities and a path to restoring his eligibility.
Howman said a reduced ban is possible depending on the level of cooperation.
"Is he trying to do something for himself to have the sanctions changed?" Howman said. "Does he want to do something for the benefit of the sport itself? In both instances, he will need to make a full statement on oath."
International Olympic Committee vice president Thomas Bach said Armstrong should provide a complete confession to USADA or WADA.
"The TV interview is not the right platform," he told the AP.
The International Cycling Union, meanwhile, urged Armstrong to testify before its independent commission on doping to shed light on allegations that include whether the UCI helped cover up his use of performance-enhancing drugs.
Cycling's governing body said it was aware of reports that Armstrong had confessed during the Winfrey interview, which will be broadcast over two nights on Thursday and Friday.
"If these reports are true, we would strongly urge Lance Armstrong to testify to the Independent Commission established to investigate the allegations made against the UCI in the recent USADA reasoned decision on Lance Armstrong and the United States Postal Service (USPS) team," the UCI said in a statement.
The UCI set up an independent panel in November to investigate the Armstrong case and what role the governing body had in the scandal. The UCI has been accused of covering up suspicious samples from Armstrong, accepting financial donations from him and helping him avoid detection in doping tests.
However, WADA and USADA each complained later on Tuesday that the UCI panel is failing to offer amnesty to potential witnesses, which will discourage people from coming forward. The three-member UCI commission, chaired by retired British judge Philip Otton, will meet in London from April 9-26, with a June 1 deadline to deliver its report.
Bach, a German lawyer who leads the IOC's anti-doping investigations, said a limited admission of doping by Armstrong will not be enough.
"This is not new," he said. "If he says in general terms that he used prohibited substances, for himself and his reputation it would come a little late. For the fight against doping, it would not help. He has to show how he managed to get around the tests and whether there was somebody who helped him."
In a statement from Montreal, Howman said WADA had followed "with interest" the reports of Armstrong's confession but that more was needed.
"While WADA encourages all athletes to come clean about any doping activities they have been involved with or know about, these details must be passed on to the relevant anti-doping authorities," he said.
"Only when Mr. Armstrong makes a full confession under oath—and tells the anti-doping authorities all he knows about doping activities—can any legal and proper process for him to seek any reopening or reconsideration of his lifetime ban commence."
In a separate phone interview, Howman said he had not been contacted by Armstrong but was open to hear from him.
"My phone number seems to be known to many people around the world and I'm sure that he could find it," Howman said. "If that day comes then I'll deal with it. I'm certainly open to dealing with anything."
Howman also questioned whether life bans were the solution to cycling's doping problems, saying that offering amnesty to riders who come forward with information would help the sport clean itself up.
"This is a very good opportunity for the page to be turned, once it has been fully written," he said. "I don't think it will be fully written unless all the stuff comes out, and to get all the stuff out there has to be consideration of some form of amnesty or forgiveness to those who have all the information."
AP Sports Writer Graham Dunbar in Geneva contributed to this report.