RIO DE JANEIRO (AFP) - Tempers frayed at the Rio Olympic swimming competition on Sunday, as athletes tore into two-time doping offender Yulia Efimova after she won her breaststroke semi-final.

The Russian, who was cleared to compete in Rio after a convoluted drugs case, triggered a chorus of boos when she was introduced and after the race as rival swimmers made their feelings known in no uncertain terms.

Defending Olympic breaststroke champion Ruta Meilutyte pulled no punches.

"It's never nice," said the Lithuanian of competing against a swimmer who has twice tested positive - the second time for meldonium, which officials ruled she could have taken before it was banned in January.

"I guess the whole point behind competition is we swimmers around the world, we train the hardest every day to be able to perform well and represent our countries and we train fair.

"And when something like that happens it's never nice and it's a little bit disrespectful," she added. "These are not the values of our sport."

American gold medal contender Lilly King, the fastest qualifier in 1min 5.70sec, echoed those sentiments.

"I'm up for the challenge and if that's what she feels she needs to do to be able to compete then you know, whatever, that's her deal," she sniffed.

"I'm here to compete clean and that's what I'm going to do," added King, admitting that beating Efimova, who qualified 0.02sec behind her, would be extra special given the Russian's murky past.

"Oh yeah, it would be a great story. Obviously, just with all the doping and things going on the past year or two, I think it would be a really great moment for us."

Efimova's case underlined the confusion caused by the recent doping scandal, which almost resulted in a blanket ban on Russian athletes in Rio following allegations of state-sponsored doping.

But swimmers in Rio have been unequivocal in their condemnation of doping offenders.

Veteran American swimmer Dana Vollmer also let rip when asked about Efimova.

"It's definitely been one of those 'she's allowed, she's not, she's allowed, she's not' and Lilly and Katie (Meili) both just prepared as if she would be there," said Vollmer, after taking bronze in the women's 100m fly.

"Everyone likes to assume athletes are clean and ready to compete and all you've got to do is dive in and trust what you've got.

"But I know that question kind of adds fire to a lot of us. Lilly's doing a good job of putting that in her racing."

The Russian men's 4x100m freestyle relay team, who finished fourth, were also jeered on Sunday. Seven of the country's swimmers were controversially reinstated at the last minute after first being barred, as the sports world dealt with the fallout of a damning report on state-sponsored doping in Russia.

Other countries were not immune from criticism. Chinese giant Sun Yang, who served a three-month suspension in 2014, was branded a "drugs cheat" by Mack Horton before the Australian stripped the defending champion of his 400m freestyle title on Saturday.