By Steve Keating

OMAHA, Nebraska (Reuters) - Swimmers at the U.S. Olympic Trials insist they are squeaky clean when it comes to doping but do not share the same confidence over the competition they could face in just over five weeks at the Rio Games.

With the Russian track and field team suspended from the Rio de Janeiro Games for systemic doping and a potentially similar sanction awaiting the country's weightlifters a cloud of suspicion lingers over all of the country's athletes.

World Anti-Doping Agency (WADA) chief Craig Reedie warned on Monday that they would call for “serious” action against Russia if a new investigation led by Canadian law professor Richard McLaren unearths more evidence of widespread state-sponsored doping.

"There is definitely a lot of talk, but, you know, I'm 100 percent confident that the U.S. Olympic Team will be 100 percent clean; there is no doubt in my mind," said Elizabeth Beisel, who will be one of the busiest athletes at trials trying to qualify in nine events.

"It is sad that some athletes, no matter what sport you're in, are choosing to dope.

"It's always going to be in the back of your mind, thinking about it."

The drugs question is one that is certain to linger over the June 26 to July 3 U.S. Trials as the Russian doping shadow creeps into the pools.

Some U.S. swimmers qualifying for Rio face the prospect of going up against a Russian who has twice tested positive for a banned substance and could find herself competing at the Olympics in August instead of serving a lifetime ban.

Yulia Efimova, a four-times breaststroke world champion, tested positive for meldonium but has had a temporary suspension lifted pending a final ruling by the Court of Arbitration for Sport (CAS).

Hundreds of athletes this year have tested positive for meldonium but WADA has said their bans might be overturned due to a lack of clear scientific information on how long the drug takes to be excreted from the body.

If her ban is held up it would be the second offence for Efimova, who was disqualified for 16 months in 2014 by swimming's world governing body after traces of an anabolic steroid were found in her system.

"No doubt in my mind that somebody that's been tested twice positive during this window of time, I don't see how would be allowed to swim in the Games," said David Marsh, head coach of the U.S. women's swim team.

"I think there is systemic (doping), but that (Russia) is not the only nation that has had patterns of behavior that seem to go beyond the norm.

"I feel very good about the U.S. Trials. I feel like this is a venue where we have people of the highest character and ethics, and I feel like we will have a very clean event here and we'll have a very level playing field."

While the doping scandal has not touched U.S. athletes directly, swimmers competing in Omaha worry their performances and results are being viewed through a doping prism.

"There is a level of frustration and anger that comes with it," said Katie Ledecky, who could become the first U.S. woman to swim the 100 metres, 200m, 400m and 800m freestyle at a single Olympics. "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."

(Editing by Larry Fine)