Armstrong, facing an imminent deadline, has indicated he may give full details about how he was able to use performance-enhancing drugs for years without detection, ABC News reported Wednesday.
"We understand that he does want to be part of the solution and assist in the effort to clean up the sport of cycling," Travis T. Tygart, chief executive of the U.S. Anti-Doping Agency said in a statement.
"We have agreed to his request for an additional two weeks to work on details to hopefully allow for this to happen," Tygart added.
Armstrong had faced a Wednesday night deadline to testify before his lifetime ban from racing became final.
The seven-time Tour de France winner is reported to be under active investigation by U.S. authorities for obstruction, witness tampering and intimidation of people allegedly aware of his cheating.
If federal charges are filed, Armstrong could face 5-10 years in prison if convicted.