Swimming's world governing body FINA has strongly criticized the organizers of next year's Olympics in Rio de Janeiro over what it says are substandard facilities and "disrespect" for aquatic events.

FINA, which oversees some of the Olympics' most-watched events, attacked the organizers and Rio mayor Eduardo Paes on various grounds including slashing the seating capacity at the main pool, according to a FINA letter obtained by The Associated Press.

Athletes' health and safety could be at risk over viruses in the area of sea used for open water swimming events and organizers' decision to use an open-air venue for diving, water polo and synchronized swimming, FINA warns.

The letter, dated Monday and addressed by FINA president Julio Maglione to Paes and the organizing committee, was circulated among leading swimming figures.

It suggests relations are at breaking point.

"The recent decisions of Mr. Eduardo Paes ... are seriously damaging the image and value of FINA and its disciplines," the letter states. "This situation is in clear disrespect for the FINA requirements concerning aquatic venues, and will negatively affect the safety conditions and the level of performances of our athletes."

Four-time Olympic swimming champion Alexander Popov is quoted in the letter as saying that swimming conditions in Rio would be "a step back in relation to previous editions" of the Olympics.

FINA is particularly critical of changes to the Olympic Aquatics Stadium, the main pool for swimming's most-watched events. FINA says design changes mean the stadium will seat only between 12,500 and 13,000 people compared to 17,500 for the main pool at the last Olympics in London.

"Therefore, the main venue of the leading sport from the Olympic movement ... is diminished in terms of importance and value," the letter states.

"Providing less available space for TV, media, athletes and spectators, this will naturally have its negative impact in terms of the coverage of the Olympic competition and in terms of working conditions for all those using the venue."

The letter demands testing for virus levels in the water off the Copacabana beach, which will be used for men's and women's marathon swimming events. Organizers agreed to test for viruses earlier this month after earlier insisting that only bacterial testing was required.

That came after the AP released a five-month independent study showing high levels of viruses at Olympic venues for sailing, rowing, canoeing, triathlon and open-water swimming. About 1,400 athletes will compete in the hazardous waters.

The FINA letter also says athletes' safety is at risk from a decision not to put a roof over the open-air Maria Lenk Aquatic Park, which will hold the diving, water polo and synchronized swimming events. All took place indoors at London 2012.

"The weather conditions may influence not only the athletes' performances, but more importantly their health and safety," the letter warns, going on to criticize Paes for refusing proposals to put a temporary cover on the arena.

FINA's criticisms attracted support from Vladimir Salnikov, the influential head of Russia's swimming federation, who said "these problems have to be solved on time" and that earlier recommendations from FINA "obviously haven't been listened to," in comments Wednesday to Russian agency R-Sport.