Reuters - The crackdown on athletes caught up in recent doping scandals is a step in the right direction for the sport, Allyson Felix says, but the American sprinter is still worried it will not be a level playing field at next month's Rio Olympics.

The 30-year-old, who won the 200m at the London Games in 2012, qualified for her fourth Olympics this month but will not be gunning for a 200-400 metres double after finishing fourth over the shorter distance at the US trials in Oregon.

Athletics has been under the spotlight due to a doping scandal that saw Russia banned from competing in the sport at the Rio Games after a World Anti-Doping Agency (Wada) report uncovered systemic abuse in the country.

Kenya's team were also in danger of missing Rio before their government passed legislation that complied with anti-doping codes.

"The number of scandals we see now just shows the real issues that are there in sports," Felix said.

"I would say that my confidence isn't high to think that there are not real issues. I would hope that the playing field is more level (but) it is very hard to deal with as a clean athlete. "You are going to the biggest race of your life and you hope that that is a clean race. But can you say that you are a 100 per cent that you are sure? I don't think that you can say that."

Felix said athletics was working hard to clean up the sport. "It is hard ... but I hope we are moving in the right direction," she added. "I think that exposing athletes that are happening now is I think is a step in the right direction."