Olympic 100m champion Justin Gatlin must serve a minimum drugs ban of four years, says the world governing body.
Gatlin has been banned after failing a drugs test in April
He struck a deal on Tuesday with the US authorities for up to eight years out instead of a life ban by agreeing to help the fight against doping.
The IAAF, which has the right to challenge the US ban if it wants, would accept eight years or less but says "four years would be the minimum".
"We want to see if it leads to other convictions," said a spokesman.
Gatlin, also the world champion, received his ban on Tuesday after testing positive for testosterone, his second doping offence, on 22 April.
If Gatlin just says 'I don't know what happened,' that's not good enough
IAAF spokesman Nick Davies
An eight-year suspension would have effectively ended his career, but it will still be difficult to return to top-level athletics after four years out.
Gatlin first failed a drugs test in 2001 when amphetamines were found in his samples at the USA Junior Championships.
But it was accepted that medicine he had been taking for 10 years to control attention-deficit disorder was the reason for the failed test.
Gatlin was suspended for two years, but the IAAF reinstated him after one year.
"We want him to tell the truth about what really happened. If Gatlin just says 'I don't know what happened,' that's not good enough," said the IAAF's Nick Davies.
"We want him to co-operate fully with USADA and be very truthful with what happened."
Gatlin's willingness to co-operate could also see him escape facing criminal charges under the ongoing FBI investigation into the Bay Area Laboratory Co-Operative, the California clinic he and Graham have been linked to.
The IAAF is currently investigating Gatlin's coach Trevor Graham for doping violations, and he has subsequently been barred from US Olympic Committee training sites because of these links.
At least nine athletes with links to Graham have been convicted of doping violations.
Marion Jones, the former Olympic champion formerly coached by Graham, failed a doping test in June at the US Championships and awaits the results of her B sample.