This follows the surfeit of doping-related controversies in the sport, including allegations this month that Russian anti-doping officials unsuccessfully requested money in return for covering up failed drug tests registered ahead of London 2012.

Particular concerns have been raised in relation to Russian and Chinese swimmers, with world champion Yuliya Efimova and Olympic gold medal winning counterpart Sun Yang among leading athletes to be implicated.

Efimova suffered the second failure of career after testing positive for meldonium in March, but has had her suspension provisionally lifted due to an admission that "more reasearch" is required to assess how long the substance - only banned on January 1 - remains in the human body.

Allegations of state sponsored doping in other sports in Russia has heightened fears in recent weeks.

"It's the biggest threat to who should win the medals," said Marsh ahead of the US Olympic Trials.

"It's the biggest threat to the integrity of the Games."

He described the effects of doping as a "complete game-changer" particularly in women's events.

"With a little bit of extra testosterone, it's a giant advantage."

Many members of US team admitted to being concerned by possible doping by rivals ahead of the Games.

"It's really disappointing," freestyle superstar Katie Ledecky told Associated Press.

"I think we're all happy that people are getting caught and they're being a little tougher on things.

"Hopefully, that will continue and we can all feel confident going in that we're competing against clean athletes."

US swimmers have been implicated in doping scandals themselves, with breaststroke and freestyle star Jessica Hardy serving a one-year ban after failing a test in 2008 before returning in time to win a medley relay gold at London 2012.